Tertiary, from carbonic esters

The Grignard reagent provides two of the three R groups attached to the hydroxylbearing carbon of the tertiary alcohol. The ester provides the third R group. So, from... 

In terms of functional group compatibility, ethers, alcohols, tertiary amines, acetals, esters, amides and heterocycles are compatible with the Pauson-Khand reaction. In the intramolecular version, relatively few carbon skeletons undergo the cyclization. Most intramolecular PKRs use systems derived from hept-l-en-6-yne (6) or propargyl allyl ethers (7) or amines (8). Other interesting and more recent substrates are enynes connected through aromatic rings like 9-11, which have allowed us and other groups to obtain aromatic polycycles (Fig. 1) [28-31]. 

The dynamic resolution of an aldehyde is also possible as shown in Fig. 15-4812801. The racemization of the starting aldehyde and enantioselective reduction of a carbonyl group by bakers yeast resulted in the formation of tertiary chiral carbon centers. The ee of the product was improved from 19 % to 90 % by changing the ester moiety from the isopropyl group to the neopentyl group. 

The first step in this scheme is a Michael addition of the nucleophile to the j5-carbon of the alkynyliodonium salt to give the ylide 102. Loss of iodobenzene from 102 gives alkylidenecarbene 103, which rearranges to alkyne 104 in the absence of external traps. This mechanism is experimentally supported by the isolation of cyclic by-products 108 besides the major products, the alkynyl esters 107 in the reaction of alkynyliodonium salt 105 with nucleophiles (equation 67). These cyclic enol ethers are the result of the insertion of the intermediate carbene 106 into the tertiary-8-carbon-hydrogen bond. 

Decarbonylation from acyl radicals is a serious side reaction in the reaction of tellurol esters bearing secondary and tertiary alkyl carbon units as R. However, decarbonylation can be avoided by carrying out the reaction under CO pressure. In addition, electrochemical oxidation of the product 102 in the presence of water afforded the corresponding a-acyl amide in quantitative yields. 

Reaction of Grignard reagents with esters (Section 14.10) Produces tertiary alcohols in which two of the substituents on the hydroxyl-bearing carbon are derived from the Grignard reagent. 

Esters react with two equivalents of a Grignard reagent to produce tertiary alcohols. Two of the groups bonded to the carbon that bears the hydroxyl group in the tertiary alcohol are derived from the Grignard reagent. 

This tertiary ester was developed to reduce aspartimide and piperidide formation during the Fmoc-based peptide synthesis by increasing the steric bulk around the carboxyl carbon. A twofold improvement was achieved over the the standard Fbutyl ester. The Mpe ester is prepared from the acid chloride and the alcohol and can be cleaved under conditions similar to those used for the r-butyl ester. ... 

Allylsilanes are available by treatment of allyl acetates and allyl carbonates with silyl cuprates17-18, with antarafacial stereochemistry being observed for displacement of tertiary allyl acetates19. This reaction provides a useful asymmetric synthesis of allylsilanes using esters and carbamates derived from optically active secondary alcohols antarafacial stereochemistry is observed for the esters, and suprafacial stereochemistry for the carbamates20,21. 

Two closely related methods for the diastereoselective preparation of <5-oxo esters have been developed. The first method uses the chelated lithio enamine 2. These Michael donors are readily available from the tert-butyl ester of L-valine and jS-oxo esters. The Michael addition of this lithio enamine 2 to 2-(arylmethylene)propanedioates, followed by hydrolytic removal of the auxiliary, provides d-oxo esters with contiguous quaternary and tertiary carbon centers with high diastereoselectivity59 60. 

In general, aliphatic diacyl peroxide initiators should be considered as sources of alkyl, rather than of aeyloxy radicals. With few exceptions, aliphatic acyloxy radicals have a transient existence at best. For certain diacyl peroxides (36) where R is a secondary or tertiary alkyl group there is controversy as to whether loss of carbon dioxide occurs in concert with 0-0 bond cleavage. Thus, ester end groups observed in polymers prepared with aliphatic diaeyl peroxides are unlikely to arise directly from initiation, but rather from transfer to initiator (see 3.3,2.1.4),... 

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