Dihydro Derivatives

There are two possible types of dihydro-furans and -thiophenes (cf. 214, 215) and examples of both are known. There are three possible classes of dihydropyrroles /V-unsubstituted 2-pyrrolines (216) are in tautomeric equilibrium with the corresponding 1-pyrrolines (217). 

The reaction of these dihydro compounds can be divided into three categories a pronounced tendency to aromatize, behavior analogous to aliphatic compounds of similar functionally, and other reactions. These are considered in turn. 

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Selective reduction of the 22,23-olefin of avermectin yields the 22,23-dihydro derivative assigned the nonproprietary name ivermectin (18). The stmcture shown depicts the 25-j -butyl derivative [70161 -11-4] but it should be noted that both commercial products contain up to 20% of the 25-isopropyl... 

Methylation of avermectins B and B2 leads to the corresponding derivatives of the A series (49). A procedure involving the oxidation of the 5-methoxy group with mercuric acetate and NaBH reduction of the 5-keto-intermediate allows the conversion of the A to the B components (50). The 23-hydroxy group of the "2" components, after selective protection of the other secondary hydroxy groups, is converted to a thionocarbonate, which can be elirninated to give the 22,23-double bond of the "1" components alternatively it can be reduced with tributyltin hydride to the 22,23-dihydro derivatives (= ivermectins) (51). 

It is customary to perform the condensation of 1,4-dicarbonyl compounds with hydrazines in the presence of mineral acid to avoid the formation of A-aminopyrroles. Contrary to early claims that 4,5-dihydropyridazines are formed <07CB4598), these compounds are now regarded as 1,4-dihydro derivatives 81CB564). 

Pyrimido[4,5- f]pyrimidines may be used as pyrimidine precursors. Thus, the dihydro derivative (736) undergoes alkaline hydrolysis to the amide (737 R = PrCO) which may be deacylated in ethanolic hydrogen chloride to give 5-aminomethyl-2-propylpyrimidin-4-amine (737 R = H) (64CPB393) rather similarly, the pyrimidopyrimidinedione (738) reacts with amines to give, for example, 6-amino-5-benzyliminomethyl-l,3-dimethylpyrimidine-2,4(lFf,3Ff)-dione (739 R = CH2Ph) or the hydrazone (739 R = NH2) (74JCS(Pl)1812). 

The fusion of a benzene ring to pyrazine results in a considerable increase in the resistance to reduction and it is usually difficult to reduce quinoxalines beyond the tetrahydroquinoxa-line state (91). Two possible dihydroquinoxalines, viz. the 1,2- (92) and the 1,4- (93), are known, and 1,4-dihydroquinoxaline appears to be appreciably more stable than 1,4-dihydropyrazine (63JOC2488). Electrochemical reduction appears to follow a course anzdogous to the reduction of pyrazine, giving the 1,4-dihydro derivative which isomerizes to the 1,2- or 3,4-dihydroquinoxaline before subsequent reduction to 1,2,3,4-tetra-hydroquinoxaline (91). Quinoxaline itself is reduced directly to (91) with LiAlH4 and direct synthesis of (91) is also possible. Tetrahydroquinoxalines in which the benzenoid ring is reduced are well known but these are usually prepared from cyclohexane derivatives (Scheme 30). 

The corresponding saturated dialkylamino ketones (Mannich bases) have also been used to provide 5,6-dihydro derivatives, which are oxidized in air to aromatic pyrido[2,3-[Pg.229]

Aluminum or sodium amalgam reduces 3-ones to 1,2-dihydro derivatives (71TH21500), as does borohydride (63JCS5156) and catalytic hydrogenation (Pd/SrCOs) (71TH21500). Catalytic (Pd/C) reduction of a 5-one also gave a 1,2-dihydro compound (74JMC553). 

Radical anions have recently been detected during electrochemical reductions of lumazines (80H(14)1603) and are also assumed to be reactive intermediates in the reductive acylation of 2,4-disubstituted pteridines to the corresponding 5,8-diacyl-5,8-dihydro derivatives. 

Little information is available about 5,6-dihydropteridines, of which various 6,7-diphenyl-5,6-dihydropterins (65HCA764, 69HCA306) and -lumazines (68HCA1029, 70HCA789) have been synthesized and characterized. As noticed already (51BSF521), this type of compound isomer-izes in an acid-catalyzed reaction to the 7,8-dihydro derivative (77HCA922) or oxidizes to... 

Reduction of isoindoles with dissolving metals or catalytically occurs in the pyrrole ring. Reduction of indolizine with hydrogen and a platinum catalyst gives an octahydro derivative. With a palladium catalyst in neutral solution, reduction occurs in the pyridine ring but in the presence of acid, reduction occurs in the five-membered ring (Scheme 38). Reductive metallation of 1,3-diphenylisobenzofuran results in stereoselective formation of the cw-1,3-dihydro derivative (Scheme 39) (80JOC3982). 

Cationic rings are readily reduced by complex hydrides under relatively mild conditions. Thus isoxazolium salts with sodium borohydride give the 2,5-dihydro derivatives (217) in ethanol, but yield the 2,3-dihydro compound (218) in MeCN/H20 (74CPB70). Pyrazolyl anions are reduced by borohydride to pyrazolines and pyrazolidines. Thiazolyl ions are reduced to 1,2-dihydrothiazoles by lithium aluminum hydride and to tetrahydrothiazoles by sodium borohydride. The tetrahydro compound is probably formed via (219), which results from proton addition to the dihydro derivative (220) containing an enamine function. 1,3-Dithiolylium salts easily add hydride ion from sodium borohydride (Scheme 20) (80AHC(27)151). 

Similar alkylations may be effected on oxygen. l-(2-Chloroethyl)imidazolidin-2-one (312) when treated with potassium hydroxide or sodium hydride underwent ring closure to the tetrahydroimidazo[2,l-6]oxazole (313) (57JA5276). This approach can be used for the preparation of bicyclic hydantoins and the corresponding dihydro derivatives of (313) using the mesylate of (312) and NaH (77JHC5U, 79JMC1030). 

A 4,5-dihydro derivative (608) was prepared by the intramolecular 1,3-dipolar cyclization of (607) (75MI41607). 

Thiol-thione tautomers have not been extensively studied, but UV and IR evidence show that 5-phenylisothiazole-3-thiol exists in the SH form. Ring-chain tautomerism of 2,3-dihydro derivatives of 1,2-benzisothiazole can occur (26a 26b) and the position of equilibrium depends very much on the solvent, physical state and nature of the substituents (69JOC919, 81KGS1209). 

Various methods have been employed to overcome this difficulty. For example, a method used by Chemical Abstracts involves naming the dihydro derivative of the heterocyclic ketone (or imine or exocyclic alkene) form, and adding the words mesoionic didehydro derivative (example 131). A similar approach, favoured by Ollis (76AHQ19)1), involves naming the corresponding cation hydroxide, with the prefix anhydro (indicating removal of the elements of water) (example 132). 

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