Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

92 NITRILE OXIDES
A synthetic approach to hyperevolutin A 421, prenylated bicyclo[3.3.1]
nonanone derivative, with an acylated phloroglucinol-type fragment, has been
described (464). Intramolecular allene–nitrile oxide cycloaddition of 422 has
been used to construct the bicyclic framework and the vicinal quaternary centers
in cycloadduct 423.
O
O
O
OH
421 422
O2N
O
OMe
O N
423
O
OMe
Δ 23 -22-Oxo steroids 424 have been synthesized via 1,3-dipolar cycloaddition
of steroidal nitrile oxides to low-molecular dipolarophiles. Cycloaddition of
2-propynyl bromide to 20-carbonitrile oxide, followed by hydrogenation of the
isoxazole derivative, gives 22-enamino-24-keto steroid. The latter has then been
converted into the target enones in several steps (465).
H
H
H
OMe
424
1,3-Dipolar cycloaddition of nitrile oxide 425 with allyl bromide followed
by hydrogenation of dihydroisoxazole derivative 426 (Scheme 1.54) gives a
pyrrol-substituted steroid derivative 427 (466).
A total synthesis of functionalized 8,14-seco steroids with Þve- and sixmembered
D rings has been developed (467). The synthesis is based on the
transformation of (S)-carvone into a steroidal AB ring moiety with a side chain
at C(9), which allows the creation of a nitrile oxide at this position. The nitrile
oxides are coupled with cyclic enones or enol derivatives of 1,3-diketones, and
reductive cleavage of the obtained cycloadducts give the desired products. The
formation of a twelve-membered ring compound has been reported in the cycloaddition
of one of the nitrile oxides with cyclopentenone and as the result of an
intramolecular ene reaction, followed by retro-aldol reaction.
O
H