THE SYNTHESIS OF 4-H-2-ALPHA-FARNESENE AND 1-H-2-ALPHA-FARNESENE

Alpha-Farnesene deuterated at C1 or C4 was synthesised by regiospecifi c deuteration of 2-geranyl-3-methylsulpholene (2). Treatment of (2) wi th butyl lithium in dimethylpropenylurea (DMPU) mediated THF resulted in deprotonation at C2. Quenching with D2O/CH3CO2D gave a mixture of d euterated sulpholenes (43-68%), predominantly 2-H-2-2-geranyl-3-methyl sulpholene (5), together with bond migrated product (25-49%). Thermal elimination of sulphur dioxide gave 4-H-2-alpha-farnesene (6)(85%) but with low deuterium incorporation (60%) and poor regiospecificity. Tre atment of (2) with butyl lithium in TMEDA mediated THF resulted in dep rotonation at C5 with minimal bond migration (1%). Quenching with D2O/ CH3CO2D yielded 5-H-2-2-geranyl-3-methylsulpholene (18)(75%) which on thermolysis gave 1-H-2-alpha-farnesene (19)(86%) with high regiospecif icity and improved deuteration (85%). Some mechanistic aspects of the alkylation of 3-methylsulpholenes are discussed.