capitulo 6, terpineol. temas orgánica básica

7/25/2019 CAPITULO 6, TERPINEOL. Temas orgánica básica

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Molecules That Changed the World

From Mesopotamia through Egypt to Greece, the ancient cultures that inhabiting those beautiful lands have celebrated pine trees not only for

their woods, but also for their bark and resin, for which they found a great many applications !n this book we shall be e"amining several

compounds of evergreens trees

Turpentine, a distillate obtained from #rs, conifers, and pines, is discussed in this chapter as a source of fragrant compounds, although it has

many other uses !n later chapters, the stories of longifolene and Ta"ol, the anticancer natural product isolated from the bark of the yew tree,

will be recounted

$ine resins can be distilled into two ma%or fractions, turpentine and rosin

The e"act composition of each of these fractions varies according to the species of tree and the region of the globe from which the viscous

resin is collected

Turpentine, a pungent clear li&uid with a bitter taste, is composed of a number of organic compounds, primarily a series of volatile terpenes

The terpene natural products, which arise from the mevalonic acid biochemical pathway, have molecular skeletons consisting of isoprene

units, isoprene being a #ve carbon basic building block Monoterpenes 'C() terpenes*, such as terpineol, are unsaturated light oils, generally

with heavy scents, that have been used since time immemorial as ingredients in paints, disinfectants, medicines, materials for religious

rituals, and perfumes

For thousands of years our ancestors have appreciated and e"perimented with perfumes in what is a continuing fascination with this lu"ury

commodity The art of blending fragances was #rst cultivated in Mesopotamia and Egypt +ubse&uently the knowledge spread to Greece and

from there on to ome These traditions continued to be practiced and honed during the Middle -ges as the -rabs became e"perts at crafting

e"otic blends of perfumes With the advent of distillation and other tecni&ues, perfumery .ourished in ennaissance !taly /owever, as

chemistry advanced and organic synthesis was born in the nineteenth century, the burgeoning chemical industry of Germany became a

center for scent production while France became a hub for perfume blending Today the coveted crown is shared between #erce rivals on

opposing sides of the -tlantic0 France, +wit1erland, and Germany vie for superiority with the relative newcomer to the art, the 2nited +tates

The %apanese are also fast becoming serious players in the art of perfumery

The esteem in which perfumes were held throughout history is illustrates by the magni#cent craftsmanship lavished on the vessels that

contained these precious essences 2ncovering these treasures of artistry and skill has allowed historians to trace a vivid record of the

appreciation of #ne fragances down through the ages and across cultures and civili1ations



!t is perhaps suprising that while these artifacts have changed dramatically over the centuries, the basic ingredients of the scents themselves

have remained essentially the same Thus, the aroma from the 3ebanese cedar 'Cedrus libani*, so highly pri1ed by the Mesopotamians and

en%oyed by Cleopatra and Marcus -ntony,is still a key component in some stylish scents of today

4arious fashions in perfumery can be traced to di5erent periods of history or geographic origins

6ut what makes a perfume7 !n old times, before ethanol could be puri#es by distillation, crude fatty e"tracts were prepared by a techni&ue

known as en.eurage, wherein delicate .oral oils were drawn out from petals These scented oils were used in cosmetics and medicine

Modern fragances are mi"tures of essential oils and synthetic aroma chemicals diluted with ethanol ad water The description of the #nal

depends on its essential oil content Thus, eau de perfume contains (89:) percent of essential oils, eau de toilette has only ;9< percent, while

eau de cologne contains less than 8 percent of the e"pensive scented oils The additional load 'and thus e"pensive* of an eau de parfum

preparation provides a stronger and longer lasting bou&uet by virtue of the higher concentrations of essential oil

Creating a perfume can be likened to the art of composing music, and the #nest e"amples contain an intricate medley of chemical

compounds that vary in molecular structure andm therefore, in aroma and volatility The perfect design yields a layered blend that ensures

the development of the scent on the skin with time The modern era for perfumery as ushered in by developments in chemistry beginning in

the nineteenth century !mprovements in techni&ues allowed th fragile active ingredients to be e"tracted from their natural sources using

organic solvents at ambient temperature for the #rst time, thus the selection of essential oils available was greatly enriched This advance

was surpassed only when, in the second half of the nineteenth century, chemists #rst began to e"plore organic synthesis as a means to

reproduce alluring natural scents

!n (<=<, 6ritish chemist William /enry $erkin, +r, made coumarin 'which smells of hay and woodru5* in the laboratory for the #rst time This

was followed by the syntheses of other naturally occurring fragances> musk '(<<<*, vanilla '(<?)*, and violet '(<?:* 3ater, camphor and

terpineol ere added to the growing list of synthetic accomplishments The (?@)s and (?:)s fostered yet another creative surge, the in.uence

of which still pervades today Thus, fragances such as the legendary Chanel AB 8 '(?@(* and oy '(?:(* set a new standard, ushering in an

era of large scale production, distribution, and consumption of #ne perfumes

Today, these items constitute an intimate part of our lives and as we en%oy them we must not forget that their e"&uisite bou&uets are a result

of the symbiotic application of scienti#c and artistic taletns +ynthetic chemists can recreate almost any known natiral scent, and in additionthey have designed and synthesi1ed a whole host of new molecules with wonderful odors to complement those provided by nature, thereby

furnishing an ama1ing library of pleasing smells Chemistry, therefore, has been pivotal to the creation of perfumes, and has assisted in

elevating them to their current popular and accessible status as items for everyone to en%oy

+imple monoterpenes e"tracted from natural sources are responsible for the characteristic smells of many plants and fruits, including citrus

fruits, pine, spearmint, and geranium



ecognition of this feature spurred an intense interest in the molecules amongst chemists, who began to study them over one hundred years

ago Terpin hydrate captured the imagination of chemists in the last decades of the nineteenth century because not only was its constitution

&uite a pu11le, but also it was readily crystalli1ed from crude turpentine in a very pure form For many years chemists 'especially William -

Tiden, Dtto Wallach, and Georg Wagner* toiled and argued, seeking a solution to the conundrum of correct formulae to represent this and

other simple related terpenes

William /enry $erkin, r, had e"perimental chemistry in his blood -long with the synthesis of coumarin, his father, William, +r, had

discovered mauveine, the purple dye that revolutioni1ed the fashion world and started an industry in the late nineteenth century William, +r,

passed his enthusiasm for the unknown on to his son, leading to William, rs ambitious attempt to synthesi1e and con#rm the structure ofvarious terpenes

!n a style resembling that still in use today for reporting a total synthesis, William $erkin, r, related his progress along his synthetic path to

terpineol, step by step, starting from ethyl cyanoacetate and ethyl :9iodopropionate

$erkins synthesis made use of then recently discovered Grignard reaction on two occasions, each time using Grignard reagent

methylmagnesium iodide The #rst such reaction involved the addition of this reagent to a ketone, forming a tertiary alcohol, which served as

a precursor for the endocyclic alkene of terpineol The second application of the Grignard chemistry converted an ester group to the tertiary

alcohol of the target, thus completing the synthesis !n the same paper, $erkin reported the conversion of terpineol into two other

monoterpenes ehydration of terpineol gave dipentene 'the racemic form of limonene, a key component in many citrus oils*, while

hydration of terpineol gave the crystalline terpin hydrate0 thus he had conclusively determined the structures of all of these related terpenes Throughout the se&uence, $erkin described the intermediates in great detail, including such physical properties as melting points, crystal

forms, color and smell For e"ample, he noted that synthetic terpineol had the same distinct smell of lilac characteristic of the natural

material This synthetic achievement is all the more impressive when one considers the relatively primitive analytical tools available in

$erkins time

The importance of the structures established by $erkins total synthesis may not have been fully appreciated until much later, when it

became apparent %ust how many important roles they ful#ll The chemistry of terpenes has been greatly advanced since the early (?))s by

many scientists, including uilio -rigoni, whose brilliant synthetic and biosynthetic contributions have helped shape the #eld E"amples of

this ubi&uitous class of natural products include all the steroids, many vitamins, and miscellaneous compounds such as the anticancer agent

Ta"ol, mentioned above !n this book you will, time and time again, come across increasingly comple" descendants of isoprene, the synthesesof which re&uire sophisticated and ingenious strategies to master their much more comple" structures $erkins synthesis delivered racemic

terpineol 'an e&ual mi"ture of both enantiomers*

Chemists would later learn tricks allowing them to synthesi1e selectively one enantiomeric form of a given substance at will +o remember

that, with $erkins synthesis of terpineol, we are still at the very beginning of our %ourney through the synthesis archives



!n (?)(, Fracois -uguste 4ictor Grignard published his groundbreaking doctoral thesis describing his studies on organomagnesium reagents

Grignard, born in Cherbourg 'France* in (<(, was part of a generation of French chemists who made a signi#cant imprint on chemical

synthesis with the invention of a number of organometallic methods, many of which are still in use today For e"ample, $aul +abatier, who

would share the (?(@ Aobel $ri1e in Chemistry with Grignard, developed the process of hydrogenation at around this time

Grignard reagents are formed from alkyl, alkenyl, or aryl halides, by reaction with metallic magnesium in ethereal solvents -lthough to this

day the e"act nature of the species formed in poorly understood, they react as nucleophiles, and are commonly employed in adittions to

carbonyl groups !n the century since the invention of the Grignard reagent, a number of similar reagent have been developed, notably

organolithium compounds, which are also often used as strong bases Grignard and organolithium reagents have also been employed asprecursors to other organometallic species 'for e"ample, organocopper reagents*, each of which has subtly di5erent reactivity

Today, the Grignard reaction is applied, in its guises, to the synthesis of many natural products, and insdustrial materials The e"ample below

shows the application of a Grignard reagent 'vinylmagnesium chloride* in 6-+Fs synthesis of the carotenoid 1ea"anthin, a compound used

as a nutritional supplement

capitulo 6, terpineol. temas orgánica básica

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