Diastereoselective osmylation and hydroboration of beta,gamma-unsaturated N,N-diisopropylamides and acid-catalyzed conversion to delta-lactones

The title reactions of beta,gamma-unsaturated N,N-diisopropylamides occur w ith useful diastereofacial selectivity. The major diol isomer from osmylati on of alkenes 1, 10, 11, and 12 in the presence of TMEDA at -78 degrees C c orresponds to the facial preference shown in transition state model 41 (R-z = H), while the opposite preference for 42 is observed with the Z-alkene 1 3. (Table 1). Hydroboration with g-BBN does not show this inversion of dias tereofacial selectivity for the Z-alkene. All of the results in Table 2 cor respond to the usual preference for a transition state such as 45. Acid-cat alyzed lactonization of the alcohols obtained in Tables 1 and 2 can be carr ied out with overall retention of configuration to afford delta-lactones. B utenolide 5 was prepared with 90% se from alcohol 2a via osmylation followe d by acid-catalyzed lactonization to 3 and elimination using SOCl2/pyridine .