Reactions between benzaldehyde or o-anisaldehyde and a series of silyloxy dienes catalysed by the chiral acyl- oxyborane (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° respectively but with low induced enantioselectivities.
(+)-Bis[3-(heptafluorobutyryl)camphorato]oxovanadium(IV) [(+)-VO(hfc)2] (2) is an effective catalyst for the hetero-Diels-Alder reaction of a variety of anthraquinone aldehydes and silyloxy dienes giving high yields of cycloadducts. The reactions all proceed with strict endo topology giving rise to cis products; this indicates that they follow a similar mechanistic pathway to that proposed by Danishefsky for reactions catalysed by the europium complexes Eu(fod)3 and Eu(hfc)3. Reactions employing the dimethoxy aldehydes (3) and (4) and the phenolic aldehydes (5) and (6) proceed with only modest degrees of enantioselection. Reaction with the benzyloxy aldehyde (7) proceeds with significantly reduced enantioselectivity indicating that the nature of a substituent at C1 can have a significant effect. Enantioselectivity is generally higher for reactions at low temperature and when toluene rather than dichloromethane is used as solvent. The cycloadduct (24) has been elaborated to the anthracene C-glycoside (28) of known configuration, thereby establishing that the hetero-Diels-Alder reaction favours formation of the 2′R,6′R enantiomer of (24). This enantiofacial selectivity has been correlated with the sense of anisochrony observed in the 1H n.m.r. spectrum of the ketone (17), a derivative of (24), in the presence of the solvating agent (S)-(+)-trifluoroanthrylethanol [(S)-(+)-TFAE]. The enone (21) has been elaborated to the anthraquinone olivose C-glycoside (33), a synthesis demonstrating the utility of hetero-Diels-Alder reactions involving silyloxy dienes with terminal silyloxy or methyl substituents for such syntheses.