Alkenyl carbamates

The chiral information of stereogenic centers in the allyl moiety of the precursor is destroyed on deprotonation. While an i/3-bound ion pair with a planar carbon frame is a chiral compound, usually rapid racemization takes place by intra- or intermolecular migration of the cation from one face to the opposite one. The sole exceptions known at present are secondary 2-alkenyl carbamates with X = dialkylaminocarbonyloxy21, in which the cation is tied by the chelating ligand, see Section 1.3.3.3.1.2. 

Table 5. 2-Oxo-3-alkcnyl Carbamates from Addition of Lithiated 1-(4-Mc-thylphenylsulfonyl)-2-alkenyl Carbamates to Chiral Aldehydes3...

Moreover, a-titanated 2-alkenyl carbamates were found to be configurationally stable both with respect to the stereogenic center15 and the double bond16. 

Table 4. (Z)-4-Hydroxy-l-alkenyl Diisopropylcarbamates from ( )-2-Alkenyl Carbamates and Aldehydes...

With titanated 2-alkenyl carbamates, the opposite regioselectivity can also be observed. Lithiated l-(4-methylphenylsulfonyl)-2-alkenyl diisopropylcarbamates, after metal exchange with chlorotriisopropoxytitanium, add to aldehydes y-selectivelylls. The less reactive titanat-ing reagent tetraisopropoxytitanium does not apparently react with these stabilized lithium carbanions, because in its presence a-selectivity is retained (Section 1.3.3.3.1.3.2.). 

The lithium-(—)-sparteine complexes, derived from primary 2-alkenyl carbamates, are usually configurationally labile even at —78 °C. During the investigation of the (ii)-crotyl carbamate 301, the (—)-sparteine complex (5 )-302 crystallized in a dynamic thermodynamic resolution process (equation 76) and stereospecific substitutions could be performed with the slurry An incorrect assignment of the configuration of the lithium inter-... 

A couple of 2-alkenyl carbamates have been deprotonated by n-BuLi/(—)-sparteine (11) (toluene, —78°C) in the presence of an excess of trimethylsUyl chloride. The ee values in the silane, formed in situ with inversion of the configuration, demonstrate the... 

The regioselectivity in the reaction of enantioenriched lithioallyl carbamates with chlorotributyl- or chlorotrimethyltin depends largely on the substitution pattern of the alkenyl residue. Those derived from primary 2-alkenyl carbamates 302 provide exclusively or with large excess the 1-substitution products 313 with inversion of the configuration (equation No (E)-ent-314 was observed. 

As was worked out with the racemic reagents, titanated ( )-2-alkenyl carbamates rac-334 and rac-335 undergo a highly diastereoselective addition to aldehydes via a Zimmerman-Traxler transition state (equation q-jjg yields are high and the... 

When employing enantioenriched l-titano-2-alkenyl carbamates 334 in carbonyl addition, the selectivity depends on the enantiomeric purity that was achieved in its preparation (see Section IV.C.l). The (ii)-crotyl derivative (R)-334a has been employed several times (equation 92)224,252,253 optically active homoaldol products 346 are easily converted into y-lactones 347 by four different pathways, which require an oxidation step (see Section IV.C.6). Appfications in target synthesis include the natural products (-b)-quercus... 

The reaction of the metalated 2-alkenyl carbamate 6, a homoenolate reagent, with 2,2-dimethylpropanal gives almost exclusively the anti-product 7 with Z configuration around the double bond 92-309 (see p 469 for configurational assignment). 

Generation of Enantioenriched, Configurationally Stable Organolithium Reagents. (1 S,2E)-1 -(N.Af-Diisopropyl-carbamoyloxy)-l-methyl-2-butenyllithium-(—)-sparteine is configurationally stable in solution and is obtained by kinetic resolution of the racemic 2-alkenyl carbamate by n-butyllithium-(—)-sparteine with >80% de (eq 4). The enantioenriched allylstan-nane, obtained on y-stannylation, was used as chiral homoenolate reagent. The methoxycarbonylation (a, inversion) yields enantioenriched 3-alkenoates. ... 

Metalated /V -diisopropyl 2-alkenyl carbamates (130) are reported to be successfully utilized to prepare, with high -y-regioselectively (equation 51), 4-hydroxyalkenyl carbamates like (131), which are useful intermediates in the synthesis of substituted 4-butanolides. ... 

It is especially remarkable that optically active homoaldoi adducts can be obtained when enantiomeri-cally pure 2-alkenyl carbamates (47 R = alkyl) are employed. Apparently the deprotonation occurs with retention of configuration and leads to configurationally stable liAium derivatives, which, after metal exchange with Ti(OPr )4, again with retention, add to aldehydes with efficient 1,3-chirality transfer coupled with enantiofacial differentiation at the carbonyl group, indicating a rigid six-membered transition state. Recently even an asymmetric homoaldoi reaction by enantioselective lithiation of prochiral primary alkenyl carbamates in the presence of (-)-sparteine was reported. ... 

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