Synthesis of [alpha]-farnesene autoxidation products and cross-conjugated polyenes : presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemistry at Massey University
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Date
2000
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Massey University
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Abstract
3-Sulfolenes (2,5-dihydrothiophene-1,1-dioxides) are well known as diene equivalents which are readily unmasked by the cheleotropic elimination of sulfur dioxide under thermal conditions. This chemistry has been used in the synthesis of conjugated triene autoxidation products of α-farnesene and previously unknown cross-conjugated polyene hydrocarbons. α-Farnesene (3,7,11-trimethyldodeca-1,3E,6E,10-tetraene) is a sesquiterpene found in the surface coating of apples. The in vivo autoxidation of α-farnesene is believed to cause superficial scald, a serious post harvest disorder of the fruit. The principal α-farnesene autoxidation product, "Anet's Trienol" (1.8a), was prepared in five steps from geraniol. The isomeric 3Z-trienol (1.8b) was also observed as a minor component (ca. 5%). Key steps involved the use of TMEDA to effect the regioselective alkylation of 3-methyl-3-sulfolene and the cheleotropic elimination of sulfur dioxide from the resultant 2,3-disubstituted-3-sulfolene. The acid catalysed hydroperoxidation of "Anet's Trienol" was achieved with anhydrous hydrogen peroxide in THF and gave the conjugated trienyl hydroperoxide (1.7a) as a single regioisomer in good yield (47%) together with traces of the stereoisomeric 3Z-trienyl hydroperoxide (1.7b) (ca. 4%). The trienyl hydroperoxides (1.7a) and (1.7b) (96:4) were cyclised efficiently under an oxygen atmosphere in the presence of "samarium peroxide" to afforded a diastereoisomeric mixture (ca. 1:1.2) of endoperoxy hydroperoxides (1.11a) and (1.11b) (85:15). Selective reduction of the hydroperoxides (1.11a) and (1.11b) gave the corresponding endoperoxy alcohols (1.12a) and (1.12b) (85:15) again as a mixture of diastereoisomers (ca. 1:1.2). Alternative syntheses of these α-farnesene autoxidation products were investigated. Three regioisomeric allylic alcohols (2.1), (2.2) and (2.3a) were prepared from geranial and, when exposed to water or anhydrous hydrogen peroxide in the presence of an acid catalyst, underwent highly regioselective oxygen transposition reactions to give "Anet's Trienol" (1.8) and the corresponding trienyl hydroperoxide (1.7) respectively as mixtures of stereoisomers. The secondary allylic alcohol (2.1) gave only the 3E-isomeric trienes (1.7a) and (1.8a), while the tertiary alcohol (2.2) and primary alcohol (2.3a) gave mixtures of the isomeric 3E and 3Z-trienes (1.7a)/ (1.7b) and (1.8a)/ (1.8b) dependant upon on the regiochemistry of the allylic alcohol starting material. E/Z isomeric ratios of transposition products indicated that intermediate carbocations did not interconvert under the reaction conditions. An analogous radical mechanism has been presented to explain the formation of minor 3Z-trienyl species formed under conditions consistent with the generation of farnesene peroxy radicals. The Stille cross-coupling of simple iodinated and stannylated sulfolene derivatives was investigated as a route to bis-sulfolenes. 3-Iodo-3-sulfolene (3.37), prepared in 4 steps from 3-sulfolene, was coupled with 3-tributylstannyl-3-sulfolene (3.33) to give the bis-3-sulfolene (3.26) in excellent yield (95%). This constitutes the first synthesis of a bis-sulfolene. Molecules of this type represent masked cross-conjugated polyenes ([n]-dendralenes). In an effort to access the higher, unknown [n]-dendralenes, the Stille cross-coupling of 3-iodo-3-sulfolene (3.37) with a variety of mono- and bis-stannanes was investigated and bis-sulfolene precursors to [4]-, [5]-, [6]- and [8]-dendralene were prepared. The utility of 3-iodo-3-sulfolene (3.37) as a coupling partner in the Stille reaction was briefly investigated and a range of other, novel cross-conjugated polyene precursors were prepared. The carbonylative Stille cross-coupling of 3-iodo-3-sulfolene (3.37) was also achieved. 3,4-Diiodo-3-sulfolene (4.15) was prepared in four steps from 2-butyne-1,4-diol and coupling with 3-tributylstannyl-3-sulfolene (3.33) yielded the first example of a tris-3-sulfolene (4.13) in 43% yield. Capillary pyrolysis (CP) was developed as a practical alternative to flash vacuum pyrolysis and proved a valuable technique for the cheleotropic elimination of sulfur dioxide from the cross-conjugated polyene precursors prepared by the Stille coupling of iodo-3-sulfolenes. Using CP, [3]-, [4]-, [5]-, [6]- and [8]-dendralene were prepared (52-89%) and their spectral characterisation was achieved. This constitutes the first general strategy for the synthesis of this poorly represented class of fundamental hydrocarbons. The synthesis of other novel cross-conjugated polyenes was achieved using CP and further demonstrated the value of this technique. The diene-transmissive Diels-Alder reaction of the [n]-dendralenes with 4-phenyl-1,2,4- triazoline-3,5-dione (PTAD) was investigated briefly and evidence for the occurrence of the theoretical maximum number of diene-transmissive Diels-Alder reactions, viz. [n-1] was obtained.
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Keywords
Isomers, Apple coating