Chiral Gold Complex Catalyzed Cycloisomerization/Regio- and Enantioselective Nitroso-Diels–Alder Reaction of 1,6-Diyne Esters with Nitrosobenzenes

An
efficient chiral gold­(I) complex-catalyzed synthetic method
that enables the cycloisomerization/regio- and enantioselective nitroso-Diels–Alder
(NDA) reaction of 1,6-diyne esters with nitrosobenzenes is described.
The sequential ring formation protocol offers access to a wide variety
of 3,5,6,8a-tetrahydro-1H-benzo­[c]­[1,2]­oxazines as a single regioisomer in yields up to 99% and enantiomeric
excess values of up to 99%. This is in contrast to the analogous NDA
reactions of the cycloisomerized 1,6-diyne ester with nitrosoarenes
in the absence of the chiral gold­(I) complex catalytic system, which
were found to give the N,O-heterocyclic product with
the opposite regiochemistry. Experimental and computational studies
based on a postulated chiral dinuclear gold species containing two
coordinated nitrosoarene molecules that undergoes an asynchronous
concerted NDA reaction with the cycloisomerized 1,6-diyne ester provides
insight into the observed product regio- and enantioselectivities.