Vinylogization

The Michael reaction is of central importance here. This reaction is a vinylogous aldol addition, and most facts, which have been discussed in section 1.10, also apply here the reaction is catalyzed by acids and by bases, and it may be made regioselective by the choice of appropriate enol derivatives. Stereoselectivity is also observed in reactions with cyclic educts. An important difference to the aldol addition is, that the Michael addition is usually less prone to sterical hindrance. This is evidenced by the two examples given below, in which cyclic 1,3-diketones add to o, -unsaturated carbonyl compounds (K. Hiroi, 1975 H, Smith, 1964). 

Torgov introduced an important variation of the Michael addition allylic alcohols are used as vinylogous a -synthons and 1,3-dioxo compounds as d -reagents (S.N. Ananchenko, 1962, 1963 H. Smith, 1964 C. Rufer) 1967). Mild reaction conditions have been successful in the addition of ],3-dioxo compounds to vinyl ketones. Potassium fluoride can act as weakly basic, non-nudeophilic catalyst in such Michael additions under essentially non-acidic and non-basic conditions (Y. Kitabara, 1964). 

The three methine carbons of the chain can be provided by 1.3.3-triethoxypropene (method C) or /S-anilinoacroleinanil, vinylog of diphenyl formamidine issued from the condensation of aniline on tetraal-koxypropane (method A). 

Two access routes are also used with the vinylog derivatives of previous compounds, for example, either 2-(4-anilinobutadienyl)thiazolium with a ketomethylene, or 4-(4-anilinobutadienylidene)ketomethylene with 2-methylthiazolium. 

Vinylogous shift of about 100 run in symmetrical dyes, convergent and nonconvergent series, and solvent effects, have also been successfully interpreted by Brooker s and Kiprianov s conceptions. 

Acyl carbonates (234), alkoxyquinones (235) as vinylogous esters, imino ethers (236), and diketene (237) react with ethyleneimine to give the corresponding acylated ethyleneimines. 

The salts (29) condense with quartenary salts of heterocycHc bases containing an activated methyl group to yield the polycarbocyanines (30), where = 4 or 5 (73). Higher vinylogous dyes (30), hexa- and heptacarbocyanines (n = 6 or 7), have been synthesized by analogous methods (106). 

Little information is available at the present time on these systems, but UV data 51JCS3204 indicate that the hydroxy forms rather than the vinylogous amide forms are favoured and these systems are true hydroxyquinoxalines and hydroxyphenazines . 

The sulfur atom can be used to initiate C—C bond formation. 2-Thio- and 4-thio-6,7-diphenyllumazine (166) react with phenacyl halides to give the phenacylthio derivatives (167), which on heating in DMF in the presence of triphenylphosphine extrude sulfur to form the benzoylmethyl derivative (168) in its tautomeric vinylogous amide form (169 equation 51). 

The reaction of vinylogous amides, or ketoaldehydes, with hydroxylamine produced 4,5,6,7-tetrahydro-l,2-benzisoxazole. A side product is the 2,1-benzisoxazole (Scheme 173) (67AHC(8)277). The ring system can also be prepared by the reaction of cyclohexanone enamines with nitrile oxides (Scheme 173) (78S43, 74KGS901). Base treatment produced ring fission products and photolysis resulted in isomerization to benzoxazoles (76JOC13). 

Nucleophilic attack on ring carbon (Scheme 39) is the most important reaction of these compounds (the electrophile may bond to oxygen either before or after nucleophilic attack). For vinylogous nucleophilic opening by 5n2 attack on ethenyloxiranes see Section 5.5.3.8. 

This vinylogous amide has been prepared in 70% yield to protect amino acid esters. It is cleaved by treatment with either aqueous bromine or nitrous acid (90% yield). ... 

Occasionally a free ketol rather than its acetate may be cleaved successfully in other cases cleavage does not occur. Starr and Smith " have summarized the available data including that covering the reduction of vinylogs of ketols. The experimental findings may be rationalized in several ways, but no firm conclusions seem possible. Smith has discussed some of the pertinent factors that must control the reduction of ketols. 

This approach has been most useful for the preparation of vinylogous a-halo ketones (see section II-H). 

Halogenation of the double bond usually prevents the satisfactory preparation of vinylogous a-halo ketones by direct reaction of unsaturated ketones with... 

The formation of derivatives of this type by free-radical attack has been mentioned previously (see section E above). The most common route to vinylogous halo ketones is by halogenation of dienol acetates or ethers. Both free halogen and A -halo compounds may be employed, and this approach has frequently been used to obtain 6 (axial) halo compounds ... 

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