By Armin de Meijere, A. Brandi, F.M. Cordero, A. Goti, T. Hirao
This paintings on small ring compounds in natural synthesis covers cycloadditions onto methylene- and alkylidenecyclopropane, and selective changes of small ring compounds in redox reactions.
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Extra info for 178 Topics in Current Chemistry: Small Ring Compounds in Organic Synthesis V
Entry I 40 =C,? , 7 days 35 °C. adduct OR 10% 83 82 8% 15% 327 84 8% 10% O_~N_.. 82 9% 6% Ref. -Spirocyclopropane isoxazolidine 324 @°. = l= O E =,. E 5" O == O o A. Goti et al. H toluene,110"C,6 h O H ~ 68% 292 1. TsNHNH2,MeOH, 60"C, 5h 2. -OH 85% (+)-Lentiginosine Scheme 47 Hydroxylated nitrones 321 and 322 afford, by subsequent thermal rearrangement of the adducts, a straightforward approach to polyhydroxylated indolizidines, inhibitors of glycosidases. The total synthesis of (+)-Lentiginosine is representative of the process (Scheme 47) .
Moreover, alkylidenecyclopropanes exhibit a substantially unpolarized double bond in striking contrast with experimental data [67b]. 8 Kcal/mole difference in favor of the transition state leading to the 4-spirocyclopropane isoxazolidine for 2,2-dimethylmethylenecyclopropane, was unable to predict the lack of regioselectivity with MCP . To explain the regiochemical outcome of the reaction, another rationale approach could be undertaken. There is still ample debate among theoretical chemists about concertedness or unconcertedness of pericyclic reactions [76-78].
The yields are only moderate, but higher with more stable nitrile oxides (Table 27, entries 5, 6, 10-12). In addition, high diastereoselectivity is observed when substituents are present on the cyclopropane ring. Only the depicted diastereomers are detected (Table 27, entries 7-9) as a result of the preferred approach of the dipole from the less hindered face of the dipolarophile ("anti" approach). The cis-diethyl substituted methylenecyclopropane 346a (Table 27, entry 11), gave quantitatively a 2:1 mixture of diastereomeric isoxazolines 357a with nitrile oxide 343, the major being that one deriving from the attack of the nitrile oxide from the side of the less bulky methyl groups .
178 Topics in Current Chemistry: Small Ring Compounds in Organic Synthesis V by Armin de Meijere, A. Brandi, F.M. Cordero, A. Goti, T. Hirao