Mode/ for Stereoinduction? lodo diol formation from allylic alcohols minor I Analysis: D. A Evans, Chem. 115, Lecture 23, Dec. 16, 1993 Ho Me l2, AgOAC ■ Prevost conditions:2 equiv k2,2 equiv AgOAC, THF,-78-40°℃ a Other conditions: I2, THF/phosphate buffer; I2, THF, aq Na2CO3 provide 1, 3-diols in very high selectivi also observed with allylic ethers(OMe, OBn, OTBS Chamberlin, Tetrahedron 1984, 40, 2297-2302 Gauche A is now more Cytovaricin Synthesis tio80:20 Diastereoselection 96: 4 I Place the medium size group(-OH)outside and the small group (-H) inside Evans. Kaldor. Jones. J. Am. Chem. Soc. 1990. 112. 7001 2, HOH/THF 10A-04-lododiol formation 10/8/00 8: 11 PM■ Place the medium size group (–OH) outside and the small group (–H) inside ■ Other conditions: I2, THF/phosphate buffer; I2, THF, aq Na2CO3 provide 1,3–diols in very high selectivity Model for Stereoinduction? major minor Gauche B is more energetically destabilizing than gauche A I2, AgOAc + + – OAc AcO– B A A B H2O + + I2, AgOAc Gauche A is now more destabilizing than gauche B OH2 Ratio 80 : 20 perfect regioselectivity HCO3– I2, HOH/THF Poor regioselectivity affords a mixture of products Evans, Kaldor, Jones, J. Am. Chem. Soc. 1990, 112, 7001. Chamberlin, Tetrahedron 1984, 40, 2297-2302. Diastereoselection 96 : 4 I2, THF aq KH2PO4 Cytovaricin Synthesis ■ High selectivities are also observed with allylic ethers (OMe, OBn, OTBS) ■ Prevost conditions: 2 equiv I2, 2 equiv AgOAc, THF, –78 →0 °C 94 : 6 98 : 2 95 : 5 80 : 20 78 90 85 Substrate Major Product Selectivity Yield (%) Iodo diol formation from allylic alcohols ■ Analysis: D. A. Evans, Chem. 115, Lecture 23, Dec. 16, 1993 HO H R C H H OAc I OH R R' OH C Me I R' HO H R C Me C HH I I Me OH Me OH Bu Bu Me R Me OH TIPSO Me Bu OH Me OH Me OAc R Bu R' OH I OH OH Bu I Me OAc C C I H OH Me OH Bu OAc OH Me R R HO OH R OH OH Me I OH R Me Me Bu Bu OH I I I I Me HO Me OAc TIPSO OH R' H H R HO C C I HO H H R' R H 10A-04-Iododiol formation 10/8/00 8:11 PM