This method has also been extended to the synthesis of mono(benzannulated)
spiroacetals [139, 140] (Scheme 66). For both the threo and erythro glycal-derived
epoxides, the retention spiroacetals 265a and 266b could be obtained selectively
using the Ti(Oi-Pr) 4 protocol. The inversion spiroacetals 265b and 266a could be
accessed upon addition of MeOH or AcOH to the reaction mixture; however, the
selectivity was much lower.
The effect of substitution on the aryl ring in this system was also studied by the
same authors [140] (Scheme 67). The retention spiroacetals 268 were obtained
under spontaneous cyclization conditions with good selectivity across most ring
sizes, except for the electron-poor NO 2 -substituted analogue. The selectivity could
be completely reversed, in some cases, using the previously developed methanolinduced spirocyclization conditions to give the inversion spiroacetals 269
predominantly.
Scheme 65 Tan et al.’s selective syntheses of spiroacetals via intramolecular epoxide ring
opening [137, 138]
Synthesis of 5,6- and 6,6-Spirocyclic Compounds
235