Alkylidene derivatives

However, benzylidene derivatives show a strong bathochromic shift in comparison with alkylidene derivatives. Thus absorption is a result of the whole conjugated system that is comparable to that of the quinoid dyes. The color of this type of compound is sensitive to acids and bases. 

Tertiary pyrrolines (49, = 1) and piperideines (49, = 2) (if R = H and the enamine can exist in the monomeric form or if R = aryl) evidently possess an endocyclic -double bond (79,155,156). The stretching frequency of the double bond can be lowered to 1620-1635 cm by conjugation with an aromatic substituent. The double bond of an analogous compound with aliphatic substituents in position 2 may occupy either the endo or the exo position. Lukes and co-workers (157) have shown that the majority of the five-membered-ring compounds, traditionally formulated with the double bond in a position, possess the structure of 2-alkylidene derivatives (50) with an exocyclic double bond, infrared absorption at 1627 cm . Only the 1,2-dimethyl derivative (51) is actually a J -pyrroline, absorbing at 1632 cm . For comparison, l,3,3-trimethyl-2-methylene pyrrolidine (52) with an unambiguous exocyclic double bond has been prepared (54). 

Alkylidene derivatives of 5-(p-dialkylamino-phenylimino)selenazol-2-one hvdra-zones, table of products. 251 4-Alkylidenedihvdropyridines. from 5-hydroxy THIS s, 10... 

A great acceleration was also observed in the cycloadditions of alkylidene derivatives of 5-iminopyrazoles with nitroalkenes, as electron-poor dienophiles, under MW-irradiation in solvent-free conditions [40c]. Some results are illustrated in Scheme 4.10. All the reactions took place with loss of HNO2 and/or NHMei after the cycloaddition, inducing aromatization of the final product. 

Alkylidene derivatives corresponding to 85 have not yet been reported in the D-ribofuranose series, as evidenced by the formation of the nitrile 82 in high yield. 

Benzyl and triphenylmethyl ethers of carbohydrates are preferred over methyl ethers when selective removal of protecting groups is important. The relatively high nucleophilic activity of the 5-hydroxyl group in glycosides and 1,2-O-alkylidene derivatives of 4 permits its benzylation and triphenylmethylation under mild conditions. Thus, treatment of 33 (Ref. 34) and 36 (Ref. 57) with benzyl bromide and... 

In the reaction of 2-isopropylidene-l-cyclopentanone 201 with carbon disulfide and ammonia, the perhydrocyclopenta[e][l,3]thiazine-2-thione derivative 204 was formed. The probable intermediates of the reaction are the Michael adduct 202 and the 2-imino-l-cyclopentanedithiocarbamic acid derivative 203. Under similar conditions, the related alkylidene derivatives gave the corresponding 3-substituted-2-imino-l-cyclopentanedithio-carbamic acid derivatives. Further substituted derivatives and homologs of 204 were also synthesized (74AKZ319), but the stereochemistry of these compounds was not investigated [73JCS(P1)1009]. 

The rate of reactions of these alkylidene derivatives was found to be sensitive to the nature of the alkylidene substituent and to the stereochemistry around the 7-alkylidene double bond. Thus, 7-isopropenyliden derivatives [406 and 407 (R = CH3)] do not react (73JA1553). 

C-5 substituent either a pyrimidine (98) (when = NR2) or an isothiazole (99) (when R = Ar) are finally obtained. Sodium hydride as base resulted in ring fragmentation to give the open-chain adduct (100). Multistep mechanisms are proposed for these transformations (89BCJ1086>. The behavior of methylthio and dialkylamino salts (5 R = SMe and NRj, respectively) is quite similar and both give with carbon nucleophiles (e.g. Meldrum s acid) the same alkylidene derivative (97) (X , = Meldrum s acid residue) <(85CC696,88JCS(P1)899>. 

Scheme 32 ALkylidene derivatives of hexuronic acid 6,3-lactone...

In the middle of the 1950s, Knott reported the synthesis of dyesmffs based on benzothiazole derivatives. Alkylation of Al-methylbenzo-l,3-thiazole-2-thione with a-bromoacetophenone and deprotonation of the resulting thiocarbonylium salt 5 yielded, after spontaneous desulfurization of the intermediate thiirane (7), the alkylidene derivative 8 (18) (Scheme 5.1). In order to rationalize the reaction, thiocarbonyl ylide 6 was proposed as the precursor of thiirane 7. To the best of our... 

This is a general method, leading to either 3-hydroxypiperazine-2,5-diones, or to the corresponding alkylidene derivatives (74CB2804). Thus A-pyruvoylproline methylamide cyclizes in water at pH 7.5 to yield the piperazinedione the cyclization appears to be highly stereoselective, leading to the kinetically controlled product (26). The scope of the method has been considerably expanded by Dutch workers, who have used it for the synthesis of l-hydroxypiperazine-2,5-dione derivatives (see Section VI). 

Earlier work had revealed [70JCS(C)980] that the 3-arylidene derivatives possess the Z-configuration. Gallina and Liberatori have confirmed this. In addition, they have isolated both Z and E alkylidene derivatives from condensation with aliphatic aldehydes. In the H-NMR spectrum, the vinylic proton is deshielded by 0.65 ppm in the Z isomer (88) compared to the E isomer (89). Other workers have confirmed these observations [80JCS(P1)419],... 

The chiral synthesis,219-221 by the use of monosaccharides, of optically active natural products having mainly branches of the B-type has now become common in organic chemistry. The reduction of alkylidene derivatives was actually used for the synthesis of thromboxane B2,222 cana-densolide,223 and a degradation product from boromycin.224... 

Ortho-esters at position 1,2- of sugars are more easily prepared than the corresponding acetals as an exchange of both functional groups is possible, 1,2-O-alkylidene derivatives can be prepared by the reaction of these ortho-esters with the appropriate carbonyl reagent in strictly anhydrous conditons and in the presence of an acid [87]. 

S. Chltfdek and J. Smrt, Oligonucleotidic compounds. V. 2 ,3 -0-alkylidene derivatives of ribonucleosides, Colt Czech. Chem. Common. 26 1301 (1963). 

Alkylidene derivatives of phthalic thioanhydride are formed as shown in Scheme 160. Reaction of phthalic thioanhydride with hot triethyl phosphite produces trafts-S -bithioph-thalide (457), probably via the carbene and phosphorane (Scheme 161) (72AHC(14)331>. Support for this mechanism stems from the fact that brief treatment of phthalic thioanhydride with triethyl phosphite in the presence of phthalic anhydride gives (458) in the presence of benzaldehyde the same reaction leads to the benzylidene derivative (456). An alternative mechanism has also been suggested, in which the penultimate step is the formation of an epoxide, which is deoxygenated to yield the product (72AHC(14)331>. 

An attractive method to generate C5-monoalkyl-substituted Meldrum s acids consists of a two-step synthesis (Scheme 10). In the first step, a Knoevenagel reaction between Meldrum s acid (22) and an aldehyde (or sometimes a ketone) yields the C5-alkylidene derivative 23. In the following step, the alkylidene derivative 23 is reduced, to generate the C5-monoalkyl-substituted Meldrum s acid 24.[43 71 104 1051 Alternatively, in the second step, Michael addition of a nucleophile to the conjugated C=C bond in the C5-alkylidene derivative generates a Meldrum s acid 25 monosubstituted on C5 by a more elaborated alkyl.t106 ... 

Scheme 10 Two-Step Preparation of a C5-Substituted Meldrum s Acid. In the Second Step, the Alkylidene Derivative Can Either Be Reduced or Undergo Michael Addition with a Nucleophile11051061...

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