Arenechromium tricarbonyls enantioselective

Wilhelm and Widdowson have exploited the asymmetric deprotonation of 470 in a synthesis of a protected version 478 of the biaryl component of vancomycin, actinoidinic acid (Scheme 185) " . One of the rings derives from an arenechromium tricarbonyl with stereochemistry controlled by asymmetric lithiation. The most readily lithiated position of 470, between the two methoxy groups, first needed blocking. Enantioselective... 

Transition-metal-stabilized carbocations can be generated from functionalized butadieneiron carbonyl or arenechromium tricarbonyl complexes [92], Reactions of such carbocations formed from chiral complexes have been studied, but low selectivities are usually observed [526, 528, 535]. However, chromium tricarbonyl complexes derived from ephedrine 5.66 suffer cyclization in acidic medium. After decomplexation, c/s-tetrahydroquinolines are formed with a high diastereo-and enantioselectivity [540,542] (Figure 5.44). 

Arenechromium tricarbonyl benzaldehydes bearing a substituent in 2-position in order to fix the conformation of the carbonyl group also undergo highly selective silylcyanation with Me3SiCN. After decomplexation, precursors of ami-noalcohols are obtained with a high enantioselectivity [539] (Figure 6.27). 

The use of chiral sulfoxitnines 1.136 (R = Ph, Tol, Y = ArCH=N) has allowed the enantioselective synthesis of p-aminoesters after cleavage of the S-N bond by CF3COOH [510], Preliminary studies showed that the reaction of C-arenechromium tricarbonyl imines and the lithium enolate of Me2CHCOOEt gave chiral p-lactams after decomplexation with an excellent enantiomeric excess [549, 1291]. 

Planar chiral compounds usually (and for the purpose of this review, always) contain unsymmetrically substituted aromatic systems. Chirality arises because the otherwise enantiotopic faces of the aromatic ring are differentiated by the coordination to a metal atom - commonly iron (in the ferrocenes) or chromium (in the arenechromium tricarbonyl complexes). Withdrawal of electrons by the metal centre means that arene-metal complexes and metallocenes are more readily lithiated than their parent aromatic systems, and the stereochemical features associated with the planar chirality allow lithiation to be diastereoselective (if the starting material is chiral) or enantioselective (if only the product is chiral). 

In the field of nucleophilic additions to arenechromium tricarbonyls [50], the chromium complex of phenyloxazohne adds various organohthium reagents re-gio- and enantioselectively in the presence of (-)-sparteine (or chiral 1,2-di-ethers). The hthium adduct thus formed has been trapped with propargyl bromide, leading to a frans-disubstituted cyclohexadiene of high ee [51] (Scheme 30). 

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