- Bromoalkoxylation of dihydropyran with bromine and 1-borneol, followed by dehydrobromination and epoxidation, afforded trans-2-1- bornyloxy-3,4-epoxytetrahydropyran as a major product. This was treated with sodium phenylselenide to give a 1:1 mixture of (2R,3R,4R)- 2-1-bornyloxy-3-hydroxy-4-phenylselenotetrahydropyran and its (2S,3S, 4S) diastereomer. Elimination of the selenoxide, derived from oxidation of the former with hydrogen peroxide, yielded the corresponding allylic alcohol, which was transformed to its vinyl ether by trans-etherification with ethyl vinyl ether in the presence of mercuric trifluoroacetate. CIaisen rearrangement of the vinyl ether provided the corresponding aldehyde which gave, after oxidation with alkaline silver nitrate and brief exposure of the resulting acid to stannic chloride, the optically pure γ-lactone (lS,6S)-3,7-dioxabicyclo[4.5.0]- non-4-en-8-one. (2S,3S,4S)-Hydroxyselenotetrahydropyran was converted to the (3R) allylic alcohol which was oxidized with manganese dioxide. Reduction of the resulting ketone with diisobutylaluminum hydride gave the (3S) allylic alcohol, from which the same γ -lactone was obtained by a stereoconvergent sequence analogous to that employed previously. Elaboration of the β-methylbutenoate side chain of monic acid C was accomplished by treatment of this γ-lactone with phenylsulfenyl chloride and isopropenyl trimethylsilyl ether in the presence of zinc bromide to yield a 1:1 mixture of the cis and trans ketoacids. The acids were esterified with diazomethane and hydroxylated with catalytic osmium tetroxide and N-methylmorpholine oxide. After protection of the diols with 1,1-dimethoxycyclohexane, the cyclohexylidene ketals were condensed with the sodium salt of tert-butyl dimethylphosphonoacetate to give the desired E-butenoate as the major product. Selective reduction of the methyl ester of these diesters with lithium n-butyldiisobutylaluminumhydride gave, after oxidation with pyridinium chlorochromate, a mixture of (2S,3S,4R,5S)-2-(3-methyl)-(E)-carbo-t-butoxypropenyl)- 3,4-cyclohexyli denedi oxy-2H-pyran-5-ylacetaldehyde and the (2R,3R,4S,5S) isomer. Coupling of the former with the γ-oxido ylide derived from (2S,3S)-3-hydroxy-2-methylbutyltriphenylphosphonium iodide furnished, after removal of the two protecting groups, monic acid C.