Experiments directed towards the synthesis of anthracyclinones. XXXIV - Hetero-Diels-Alder reactions using chiral boron and titanium reagents

Reactions between benzaldehyde or o-anisaldehyde and a series of silyloxy dienes catalysed by the chiral acyloxyborane (CAB) complex (4) give high yields of enantioselective products from a Mukaiyama aldol rather than a hetero-Diels-Alder reaction. Attempts to effect a similar catalytic reaction with anthraquinone aldehydes were unsuccessful but use of 2 equiv. of the CAB complex (1) followed by cyclization promotes a formal hetero-Diels-Alder reaction between the aldehyde (7) and the diene (12) to give the dihydropyrone (24) in 45% yield and with a 79% e.e. in favour of the 2'R enantiomer. Hetero-Diels-Alder reactions between the aldehyde (7) and the diene (12) using the chiral titanium complexes Ti[(R)-BINOL]Cl-2, Ti[(R,R)-TADDOL]Cl-2 and Ti[(R)-BINOL](2) have been investigated. The first two complexes promote the reaction at -30 and -78 degrees respectively but with low induced enantioselectivities.