FORMATION OF HOMOALLYL ALCOHOLS AND 4-CHLOROTETRAHYDROPYRANS FROM ALLYL-STANNANES, ALDEHYDES AND TICL4 OR CP2TICL2

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 \u2022 OEt2 in CH2Cl2 at 1278\ub0C 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 \u2022 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-EtCHClCHCH=CH(CH2)2CH2 (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-EtCHClCHCH=CH(CH2)2CH2 (78%) were obtained