Esters enoate

In Entry 13, the dioxinone ring undergoes thermal decomposition to an acyl ketene that is trapped by the solvent methanol. The resulting (3-keto-y,8-enoate ester then undergoes stereoselective cyclization. The stereoselectivity is controlled by the preference for pseudoequatorial conformations of the C(6) and C(9) substituents. 

Diels-Alder Reactions. (i )-Pantolactone is one of the most effective chiral auxiliaries for preparative scale Diels-Alder additions of simple enoate esters in the presence of Lewis acids (eq 1). ... 

Substrate control has also been realized for the intramolecular cyclopropanation of 2-diazo-3-oxohept-6-enoate and -oct-6-enoate esters derived from naphthylbomeol which is readily prepared from (-l-)-camphor. The diastereomeric excess, and for the hept-6-enoates even the preference for one or the other diastereomer, can be partially controlled by the catalyst. As an example of the oct-6-enoate, intramolecular cyclization of 28 gives the bicyclo[3.1.0]hexanes 29 a and 29... 

In one case the reaction was conducted as a genuine Diels-Alder reaction on the silyl enol ether 45 of the a -enolate 44 to see what the result would be.9 The Diels-Alder reaction 47 with the enantiomerically pure enoate ester 46 required 160 °C and gave a 40% yield of a 1 1 mixture of diastereoisomers of 49. 

Ligands of similar size but increasing basicity, for a given series of palladium acetate tertiary monodentate phosphines solubilized in isoquinoline, lead to improved nondi-enoate ester yields in the order ... 

A -Aryl-iV -phenylalkynylamidines are cycitzed to quinolines by treatment with hot PPA the main product is the 2-(A -methyl-yV-phenylamino)quino]ine but cyclization on to the other benzene ring may (xxur to some extent. 3-Phenyl-aminobut-2-enoate esters are cyclized (on to a ring-carbon) at or below room temperature by the application of Vilsmeier reagents. 

There are many other enantiomericaUy pure boron-based Lewis acid catalysts used in the Diels-Alder reaction, Amongst these the dichloroborane (8.40) used by Hawkins is noteworthy because the substrates used were enoate esters, such as methyl acrylate (8.41) ° a, 3-unsaturated ketones and acid chlorides such as (8.42) rather than the enals generally tested in Diels-Alder reactions. 

In the osmylation of 1,2-disubstituted allylic alcohols and derivatives, cw-alkenes provide higher diastereoselectivity than the corresponding fran -alkenes (eqs 7 and 8). Opposite se-lectivities have been observed in the osmylation of (Z)-enoate and ( )-enoate esters (eqs 9 and 10). High selectivity has also been observed in the osmylation of 1,1-disubstituted and ( )-trisubstituted allylic alcohols and derivatives and bis-allylic compounds (eqs 11-13). 

J. Martel), who reacted the senedoate (3-methylbut-2-enoate) ester with an allyl phenyl sulfone. [77-81]... 

The use of nonstabilized carbon nucleophiles in this reaction has been rare. Recently, however, it was shown that lithium ester enolates participate in Pd-cata-lyzed 1,4-additions to cyclic and acyclic vinyloxiranes, affording the corresponding 6-hydroxy-4-enoates in good yields and with complete regioselectivity [117, 118]. 

Several modified phosphonoacetate esters show selectivity for the Z-enoate product. Trifluoroethyl,256 phenyl,257 2-methylphenyl,258 and 2,6-difluorophenyl259 esters give good Z-stereoselectivity with aldehydes. The trifluoroethyl esters also give Z-selectivity with ketones.260... 

This study also gives a good account of the stereoselectivity of the 2-diazobut-3-enoate addition reaction with styrene. There is a preference for the ester group... 

Motherwell and Whitehead et al. reported a similar intramolecular reductive aldol reaction of aldehyde-enoate derivatives. The cyclization of 6-oxo-ester 23 was catalyzed by RhCl(PPh3)3 (1 mol%) with Et3SiH (210 mol%) as terminal reductant (Scheme 9) [17,18]. The cyclization proceeded at 50 °C for 18 h to give the aldol product 24 in 81% yield with ds-selectivity (cis trans =... 

Wolff rearrangement of a-diazoketones to give ketenes or subsequent products is an often used synthetic procedure the scope and limitations of which are well established 13 390), so that only a few new features of this reaction need to be considered here. Concerning its catalytic version, one knows that copper, rhodium and palladium catalysts tend to suppress the rearrangement390). A recent case to the contrary is provided by the Rh2(OAc)4-catalyzed decomposition of ethyl -2-diazo-3-oxopent-4-enoates 404 from which the p,y-unsaturated esters 405 are ultimately obtained via a Wolff rearrangement 236). The Z-5-aryl-2-diazo-3-oxopent-4-enoates undergo intramolecular insertion into an aromatic C—H bond instead (see Sect. 4.1). 

Elaboration of triol 88b to bryostatin 7 requires chemoselective hydrolysis of the Cl methyl ester in the presence of the C7 and C20 acetates, macrolide formation, installation of the C13 and C21 methyl enoates, and, finally, global deprotection. The sequencing of these transformations is critical, as attempts to introduce the C21 methyl enoate to form the fully functionalized C-ring pyran in advance of macrolide formation resulted in lactonization onto the C23 hydroxyl. In the event, trimethyltin hydroxide promoted hydrolysis [73] of the Cl carboxylate of triol 88b, and subsequent trie thy lsilylation of the C3 and C26 hydroxyls each occurs in a selective fashion, thus providing the seco-acid 89. Yamaguchi macrolacto-nization [39] proceeds uneventfully to provide the macrolide 67 in 66 % yield (Scheme 5.14). 

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