Stereochemical aspects of flavour biogeneration through baker's yeast mediated reduction of carbonyl-activated double bonds
The baker's yeast mediated reduction of the unsaturated carbonyl compounds 4, 5 and 6 has been studied in the context of the generation of the natural forms of the flavour materials 1, 2 and 3. The stereochemical aspects of these processes and of those regarding the reduction of the structurally related enones 18, 19, 3, 31 and 32 have been elucidated also by means of deuterium labelling experiments. Among the 6.000 chemically defined molecules identified up to now in trace amounts in food, whose function is aromatic rather than nutritional, nearly 1.OOO are used by industry in formulations. About 85% of the industrially needed flavour materials are obtained by chemical synthesis, whereas the reminder are mainly obtained by extractive manipulation of plant materials or by fermentation. Recently there has been legislative discrimination (1) between chemically identical food constituents of synthetic origin and those derived from natural sources. It has, therefore, become desirable to produce quantities of these flavour agents required from the food flavour industry and not available by extraction through enzymolysis (1) of natural products, using either isolated enzymes or whole cells.(2) Increased value is indeed added upon the aroma components produced through these means, because they can be labelled 'natural' thus receiving consumer preferences. This circumstance offered new challenging possibilities of application of enzymes in organic synthesis. There is, however, a peculiarity which makes this research more stimulating. At variance with usual organic transformations, where enzymes can be used as alternative to well established a-biological techniques, in the production of 'natural' flavours from natural precursors the involvement of enzymes becomes compulsory. Moreover, a major problem faced in this field concerns the steric outcome of the transformations, due to the dependence of the sensory response of chiral flavours materials from the enantiomeric composition. Since our interest concerned principally the mode of formation of C-6--C-12 y and 6lactones(3) we planned the biogeneration of 6-decanolide 1 and y-nonanolide 2, two compounds of key importance in fruit aroma formulations occurring in Nature in trace amounts in many fruits, and of 4-(4-hydroxyphenyl)-butan-2-one (raspberry ketone) 3, the impact flavour of raspberry fruit,(4) from the corresponding unsaturated compounds 4, 5 and 6, respectively, by means of baker's yeast mediated saturation of the carbonylactivated double bond. Indeed, the unsaturated lactones 4 and 5 are obtained by extraction from Cryptocuriu massoiu(5) and from Dipteryx odorutu Willd. (Tonka