conversion in 3–5 h (Scheme 51). Regioselectivity is, of course, not a problem as
both 6-endo-dig and 5-exo-dig cyclizations will give the same product, 222.
Xue et al.’s studies on the gold(I)-catalyzed spirocyclization of bis(aromatic)
alkynes 223 [111] found that addition of AgOTf was required to suppress formation
of the undesired benzofuran side product 225 (Scheme 52). In the absence of the
silver(I) salt, benzofuran 225 only was obtained in excellent yield (95 %). Electrondonating groups at R
1 afforded the spiroacetal 224 in good yield (62-68 %), while
electron-withdrawing groups at either R
1 or R
2 resulted in lower yields of the
spiroacetal. In one case (R
1
¼ H, R
2
¼ OMe), benzofuran 225 was obtained in
comparable yield to the spiroacetal product (42 % and 45 %, respectively).
Scheme 50 Deslongchamps et al.’s Hg(II)-catalyzed synthesis of mono(benzannulated)spiroacetals [109]
Scheme 51 Messerle et al.’s Rh(I)- and Ir(I)-catalyzed synthesis of bis(benzannulated)-spiroacetals
Scheme 52 Xue et al.’s gold(I)-catalyzed synthesis of bis(benzannulated)-spiroacetals [111]
Synthesis of 5,6- and 6,6-Spirocyclic Compounds
227