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

350 NITRONES: NOVEL STRATEGIES IN SYNTHESIS
595 + D7h
HO
HO
H
exo-syn
H
O
O N
O
O N
endo-anti
619
621
O
H
H
O
H
H
OBu t
OBu t
HO
O
O
O OBu
N
t
H
H
+ H +
exo-anti
HO
H
exo-anti
620
O
O N
620
Scheme 2.271
O
H
H
OBu t
+
HO
HO
H
O
O N
endo-anti
H
exo-anti
O
O N
621
O
H
H
O
H
H
622 ent
OBu t
OBu t
partial racemization did occur and consequently adduct (622 ent), derived from
D7 h ent, was formed (Scheme 2.271) (785).
Contrary to additions involving δ-lactones, where only the exo approach of the
reactants was observed, γ-lactones reacted with nitrones in exo-andendo-modes.
High preference of anti-addition to the terminal hydroxymethyl group in lactone
D7 h andtothe3-tert-butoxy group of the nitrone was observed. In these reactions,
the 4-tert-butoxy substituent plays a secondary role. The endo addition of
the reactants is energetically more demanding than the exo addition, and occurs,
if the substituents, present in the lactone or nitrone, impede such an approach.
Because of complex steric interactions only a single product (623) or(624) was
formed in reactions (595ent)/D7 h and (614)/D7 h (Scheme 2.272). In one case,
(614)/D7 g,a high preponderance toward the formation of a single adduct was
observed (785).
The reaction of morphantridine N -oxide (26) with 5-ethoxy-3-p-tolylsulÞnyl-
2-furan-(5H )-one (D7i) yields a mixture of three adducts (625) (Scheme 2.273).
The major one, anti-625-endo, when reactions were performed at ambient temperature,
was obtained in a low diastereomeric ratio: the relative proportion
of anti-625-endo, syn-625-endo and anti-625-exo was 43:36:19. However, the
relative proportion changed signiÞcantly (72:19:9) when the reaction was conducted
at 100 ◦ C. Finally, anti-625-endo was obtained in 72%, isolated, yield. The