COMPARISON OF THE ALLYLATION REACTIONS OF ALDEHYDES USING ALLYLSTANNANES WITH BORON-TRIFLUORIDE ETHERATE AND BORON-TRICHLORIDE

Reactions between allylstannanes, R2CH=CHCHR1SnBu3 (R1 = R2 = H (4); R1 = H, R2 = Me (5); R1R2 = (CH2)3 (6)) and aldehydes, RCHO (e.g. R = Et) in the presence of BF3 • OEt2 in CH2Cl2 at −78°C produce stereoselectively erythro-RCH(OH)CHR2CH=CHR1 (with one equivalent RCHO) and 4-OH-3-R1-5-R2-2,6-R2-tetrahydropyrans (with an excess of RCHO). In contrast, when BCl3 is used in place of BF3 • OEt2 the reactions give mixtures of chlorinated alkenes (both homoallyl chlorides and allyl chlorides) and 4-Cl-3-R1-5-R2-2,6-R2-tetrahydropyurans (3; X = Cl). Thus 5, EtCHO and BCl3 (all equimolar) provide EtCHClCH2CH=CHMe (51%, (E) + (Z)), EtCHClCHMeCH=CH2 (7%, erythro + threo), EtCH2CH=CH-CHMeCl (30%, (E) + (Z)) and 3 (12%, X = Cl); with EtCHO (2.2 equivalents), 3 (X = Cl; cis/trans = 70/30) becomes the sole product. The product, erythro-EtCHCl H2 (97%) was produced from equimolar EtCHO, BCl3 and 6; with excess EtCHO (2.2 equivalents), 9-Cl-2,4-Et2-cis-3-oxabicyclo[3.3.1]nonane (17%; cis/trans = 45/55) and erythro-EtCHCl H2 (78%) were obtained.