N-sulfinyl ureas

In 2002 the same author demonstrated the usefulness of this method in a rather demanding context including an intramolecular cycloaddition with an W-sulfinyl urea as a new type of N-sulfinyl dienophile (Scheme 60) [144]. As key steps in the total synthesis of freshwater cyanobacterial hepatotoxins, ( , )-diene 238 was transformed into N-sulfinyl urea 239 which immediately cycloadds intramolecularly yielding tricycle 240 as a single isomer in excellent yield. After reaction with phenylmagnesium bromide the intermediate allylic sulfoxide rearranges cleanly to diastereomerically pure allylic alcohol... 

The group of EUman described recently the highly enantioselective and diastere-oselective addition of cydohexyl Meldrum s acid to a,P-disubstituted nitroolefins, using N-sulfinyl ureas, to give anti adducts 67 [60]. Unlike Wennemers catalyst 58, Elhnan s catalyst 68 acts through non-covalent activation. Indeed, the sulfinyl... 

N-sulfinyl (thio)urea catalysts 335 N-sulfinyl ureas 1029 sulfinylation 1256 sulfonamides 335 -aryl 1299... 

Figure 6.63 Design concept for the development of bifunctlonal N-sulfinyl (thio)urea derivatives.

The N-sulfinyl (thio)ureas are modular and easily accessible in one step by condensing tert-butanesulfinamide with the appropriate isocyanate or isothiocyanate. Figure 6.64 shows a representative selection of the prepared N-sulfinyl (thio)ureas evaluated for their catalytic activity in the aza-Henry (nitro-Mannich) reaction of N-Boc-protected benzaldimine and nitroethane producing adduct 1. 

Figure 6.64 Representative N-sulfinyl (thio)ureas evaluated for catalytic activity in the asymmetric aza-Henry reaction of N-Boc-protected benzaldimine with nitroethane affording model product 1.

A similar reaction mechanism could be assumed for the aza-Henry reaction catalyzed by urea 38 [46], In this particular case, the sulfinyl group acts both as an acidifying agent and a chiral controlling element, and allows the stereoselective addition of nitroethane to aromatic and, in two instances, aliphatic N-Boc imines (Scheme 29.21). 

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