AMMONIUM YLIDES AS BUILDING BLOCKS FOR ALKALOID SYNTHESIS - Modern Tools for the Synthesis of Complex Bioactive Molecules - page 462

Functional group manipulation led to sulfonamide 281 that was isomerized by the
action of LiHMDS to give the bridged bicycle 282 in 70% yield. Reduction of the
ketone and elimination provided an enolate that underwent a base-promoted intra-
molecular conjugate addition and sulfonyl cleavage to give 283 in 32%yield from 282 .

13.2.6.3. Nonstabilized Azomethine Ylides Livinghouse and coworkers
coupled their isonitrile method with a subsequent azomethine ylide cycloaddition
in an approach toward the synthesis of the erythrina skeleton [100]. Reaction of
isonitrile 284 with acid chloride 285 gives an intermediate

-keto imidoyl halide that
undergoes acylnitrilium cyclization upon exposure to silver ion, furnishing 286 in
87% yield (Scheme 13.54). Reaction at the nitrogen atom with trimethylsilylmethyl
triflate followed by treatment with CsF generated ylide 287 , which underwent
cycloaddition with the pendant alkene to give 288 in 70% yield. Similarly, imine
289 produced 290 in 42% yield. Both 288 and 290 have the full skeleton of the
erythrina alkaloids, generalized by structure 291 .
Pandey and coworkers adapted adapted the the AgF-mediated oxidation route
to azomethine ylide generation (cf. Scheme 13.36) for a formal total synthesis of the
Amaryllidaceae alkaloid (

a

)-pancracine [101]. In this synthesis, amine 292 was
exposed to AgF to generate 293 , which then underwent cycloaddition to the proximal

O

1.

Cl

MeO

MeO

285

N

N

MeO

MeO

2. AgOTf

C

87%

O

284 286

TMSCH 2 OTf
CsF

MeO MeO

N N

MeO MeO

H

O O 70%

288 287

MeO

MeO

TMSCH 2 OTf
CsF

N

N

O 42%

O

289 290

MeO

N

MeO

RO

Erythrina alkaloid skeleton 291

SCHEME 13.54