By Mellor J.W.
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This clearly shows that activation of the oxabutadiene is caused by a kind of conjugation with the phenyl group over the sulfur atom . However, one has to keep in mind that the phenylthio group is only a weak activating moiety compared to electron-withdrawing groups such as CN and CO~R (Fig. 2-34). On the other hand, in a series of papers we have shown that 1-oxa-1,3-butadienes 2-130 bearing an electron-withdrawing group such as CN, CO2R, CC13, CC1F~ and CF3 at the 2-position also express a good reactivity, which however, is 33 Hetero Diels-Alder Reactions in Organic Chemistry toluene a: R = OMe" 120 ~ 72 h b:R=SPh:125~ 130h NPht 9 9 HaC ~ NPht PhS~ PhS ~__ + 82- 95% 2-120 NPht H3C" "O" "R 2-122 2-121 H3C~" "O" "'"R 2-123 2-122a: Raney-Ni,/ MeOH//87% ,/ I 2-122a: 85% Raney-Ni, THF NPht(H) NPht H3C~OMe H3C~"O~OMe 2-124 2-125 Fig.
3-17) . Ghosez et al. could also achieve high asymmetric inductions by reacting electron-rich 2-aza-l,3-butadienes with a, fl-unsaturated chiral oxazolines . Other applications of electron-rich 2-aza-l,3-butadienes in normal electron demand aza Diels-Alder reactions have been aimed at the preparation of natural cibrostatines  and azaanthraquinones . A less usual type of 2-aza-l,3-butadienes is the class of C=C-conjugated carbodiimides like 3-59. g. 1,3-oxazine 3-60 (Fig. 3-18) [286-288].
The lowest enantioselectivity was obtained with benzaldehyde and the best with phenylacetaldehyde and some aliphatic aldehydes to give the corresponding dihydropyrans with ee values ranging from 75 % up to 97 %. Good enantioselectivities were also found by Togni  using a novel optically active oxovanadium(IV) complex 2-72 bearing camphor-derived 1,3-diketonato ligands. 5 % de and 85 % ee. This was clearly the best result, other examples were less satisfying (Fig. 2-20). Also good results were obtained by ]orgensen  using a chiral copper(II) complex 2-77.
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