Esters alkynes

Alkynic esters react with nitrile oxides in a pH dependent reaction to product isoxazolin-5-ones (Scheme 145) (71JCS(C)86). Alkynic ethers also react with benzonitrile oxide to produce an isoxazole-ether which on treatment with HCl or HBr gave an isoxazolinone (Scheme 145) (63CB1088,58MI41600). The reaction of benzonitrile oxide with dimethoxyketene yielded a dimethyl acetal which was split with acid into the isoxazolinone (Scheme 145) (59G15H). 

Chlorination of sodium cyanodithioformate gives 3,4-dichloro-5-cyanoisothiazole (227), probably via the isothiazolodithiine (226) (72AHC(l4)l). The thienoisothiazole (228) undergoes cycloaddition with alkynic esters to give adducts such as compound (229), which... 

Section 180A (Protection of Ketones). Also via Acetylenic Esters Section 306 (Alkyne - Ester). 

Carboxylic acids can be protected as oxazolines [96, 105-107, 186, 191] or as ester functions. Alkynic esters such as silyl esters [153, 211], tert-butyl esters [216], and even benzyl esters [153, 211] have been successfully hydrozirconated when the reactive site was a terminal alkyne or vinyl group (Scheme 8-27). 

Such reactions occur readily with alkynic esters, but the products isolated are often complex. Thus, the initial Michael adduct of type (68) from pyridine with acetylenedicarboxylic ester reacts with more alkynic ester to yield (66), (67) and other products. Quinoline and isoquinoline react similarly. 

Cyclopenta[c]quinolizines (284) reacted with alkynic esters in boiling nitrobenzene to give cyclopenta[cflf][3.3.3]cyclazines (285), the alkoxycarbonyl groups of which could be readily removed by alkaline hydrolysis followed by vacuum pyrolysis of the resulting acids (69JCS(C)239>. 

In what is essentially a modification of the previous general route, an active methylene compound reacts with an alkynic ester to give a pyran-2-one. This time, however, different substituents may be introduced at C-4 and C-6. Again, this early synthetic route (1899JCS245) has found wide application which may be expressed in general by Scheme 92. 

The products of the reaction of 4-hydroxy-6-methylpyran-2-one with alkynic esters show some dependence on the solvent used. Dimethyl butyndioate yields the bicyclic pyranone (325) in the absence of solvent and using Triton B as the catalyst. Michael addition is envisaged to yield the intermediate dione (324) which cyclizes in the normal manner... 

The conjugate addition of organocopper reagents to ot,(3-alkynic esters, ketones, aldehydes and acids is a useful method for the preparation of various tri- and tetra-substituted alkenes,87 although addition to aldehydes is less common.88 Use of a vinylcuprate results in the formation of a conjugated dienone, which further highlights the importance of this methodology (equation 43 ).87... 

Early work in this area89,90 established that the conjugate addition of cuprates to alkynic esters, in THF, occurred stereoselectively at -78 C via cis addition. If the reaction is carried out at 0 C, a mixture... 

Highly stereoselective cis additions of organocuprates to alkynic esters have also been reported with alkylcopper-Lewis acid complexes.3 The choice of the proper Lewis acid was found to be important in determining the stereoselection as was demonstrated with enyne (32 Table 2).5- 8 While both the diorga-nocuprate and the alkylcopper-BF3 complex gave mixtures of stereoisomers, the Bu u-BEts provided... 

When alkynic esters are used as substrates in the reaction sequence, activated alkenes of good isomeric purity can be prepared. Initiation of the difunctionalization usually is by means of organocopper-medi-ated 1,4-addition, which proceeds via carbometallation of the alkynyl moiety of the ester to give a vi-nylcopper intermediate, (14). Trapping by an electrophile affords cis-difunctionalized alkenoates... 

One of the characteristic reactions of the Ti reagent is the intramolecular nucleophilic acylation of the titanacycle with esters, and 2,7- and 2,8-enyne esters afford interesting mono- and bicyclic skeletons [125], Titanacycle 297, formed from a, /i-unsaturated ester 296, is converted to the alkenyltitanium 300 by protonation, and the cyclopentenone 301 is formed by intramolecular acylation [126], As 297 is a tautomeric form of the Ti enolate, it reacts with electrophiles such as aldehydes to give 298, which cyclizes to 299. The a,/i-alkynic ester 302 generates the titanacycle 303 and is converted to the bicyclo[3.1.0]hexane system 305 via 304 [126]. The titanacycle... 

In order to side-step potential problems with hydroxyl-directed hydro-metallation of the alkyne, ester 14 was reduced with sodium borohydride in methanol. This also led to a small amount of alkyne desilylation (5%-10%). Fortunately, however, this was the next step of the synthesis. After selective protection of diol 15 to obtain 8, the stabilised acetylenic anion... 

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