First highly regio- and diastereoselective [3+2] cycloaddition of chiral nonracemic alkenyl Fischer carbene complexes with diazomethane derivatives: Preparation and synthetic applications of enantiomerically pure Delta(2)-pyrazolines

The [3+2] cycloaddition of alkenyl Fischer carbene complexes 1 and 6 with d iazomethane derivatives to give Delta(2)-pyrazoline carbenes 2, 4, and 7 is described. The conversion of (-)-8-phenylmenthol-derived carbenes 6 into p yrazoline esters 9 through a one-pot cycloaddition-protection-oxidation is presented as an expeditious route to enantiomerically pure 3-alkoxycarbon-y l-Delta(2)-pyrazolines in good yields and in a highly regio- and diastereos elective manner. Pyrazolidines 13 were synthesized in excellent overall yie lds from 9 through pyrazolines 11 and 12, the diastereoselective reduction of C=N bond being the key step. Azaprolines 16 and 17 were prepared in very good yields from epimeric trans-13 or cis-13 by oxidative deprotection of the silylated hydroxy group. In an alternative reaction, the pyrazolidine r ing of 13 was reduced to obtain protected open-chain 2,4-diamino alcohols 1 9 with three chiral centers.