Diels-Alder To Form 6-Atom Tether Ring MeLi 72%, 2 steps 190c KEH.o 5%, 2 steps R=Me or ph exception REMO CO2Et Note: Intermolecular variant gives xylene 1: 1 ratio with opposite Diels-Alder To Form 7-Atom Tether Ring Note: With silyl ether tether Toluene MeLi 25%, 2 steps shorter tether versions ratio not determined However 1 isomer 1 isomer Shea, K. J. Zandi, K S: Staab. A. J: Carr R. Tetrahedron Let. 1990, 31 5885 33A-063/149611:22PMO Si R R Si O R R TMS OH OH OH O Si O R t-Bu t-Bu CO2Et R O O Si EtO2C O O Si EtO2C R t-Bu t-Bu O Si O R CO2Et t-Bu t-Bu O Si O R CO2Et t-Bu t-Bu exception: TMS 72%, 2 steps 75%, 2 steps Shea, K. J.; Zandi, K. S.; Staab, A. J.; Carr. R. Tetrahedron Lett. 1990, 31, 5885. O Si Me Me Si O Me Me OH O Si O CO2Me 190 °C R = Me or Ph cis:trans = 1:1 Ph Ph MeLi R = Me KF, H2O2 R = Ph Note: In general, yields are good and regioselectivity is absolute, but stereoselectivity is poor. O Si O Diels-Alder To Form 6-Atom Tether Ring: ‡ ‡ Ph Ph CO2Me H vs. xylene 160 °C >90% R = Me or H R = H R = Me 90 10 >99 <1 O Si O : : Gillard, J. W.; Fortin, R.; Grimm, E. L.; Maillard, M.; Tjepkema, M.; Bernstein, M. A.; Glasser, R. Tetrahedron Lett. 1991, 32, 1145. Ph Ph Toluene 190 °C MeLi O Si O ratio not determined 25%, 2 steps Note: With monosilyl ether tether, reaction is slow and poor yielding in comparison to shorter tether versions. Ph Ph CO2Me H 1 isomer However: Diels-Alder To Form 7-Atom Tether Ring: 175 °C 98% 175 °C 77% 1 isomer CO2Me Note: Intermolecular variant gives 1:1 ratio with opposite regiochemistry. Note: Yields are generally high and selectivities good for 5-member silyl acetal tethers. In the case of type II Diels-Alders, regioselectivity and stereoselectivity are absolute. 33A-06 3/14/96 11:22 PM