Dimethylformamide ring closures with

Direct transformation of thiazoline-azetidinones 2 into 3 -thio-substituted cephalosporins 6 has been performed by ring opening of the thiazoline moiety with sulfenyl chloride followed by ring closure with ammonia in dimethylformamide and simultaneous displacement of the allylic chlorine atom with the leaving thiolates. 

Dimethylformamide Multi-component reactions Ring closures with enamines 2-Thiazolidones... 

The enamine 141 can be cyclized to the [l,2,4]triazolopyridopyrimidine 142 upon treatment with sodium ethoxide (Scheme 40) <2002M1297>. This fused tricyclic system may also be obtained, like the pteridine analogue (cf. Scheme 38), from the reaction of hydrazonoyl halides and pyridopyrimidines such as 143, and also by treatment of the triazolopyrimidine 144 with dimethylformamide dimethylacetal (DMF-DMA) dimethylacetal and subsequent ring closure <2003MOL333, 2003HAC491> (Scheme 41). Another series of triazolopyridopyrimidines, for example, 146, can be prepared from a hydrazine-substituted pyridopyrimidine 145, in two ways either directly by reaction with an acid chloride, or via a derived hydrazone (Scheme 42) <1996MI585>. 

Quite analogous ring-closures occur when the 1-O-acetyl derivatives of the rhamnopyranose and talopyranose derivatives are treated with sodium azide in N,N-dimethylformamide. l-O-Acetyl-6-deoxy-2,3-0-isopropylidene-4-0-mesyl-a-L-mannopyranose is converted exclusively into l,4-anhydro-6-deoxy-2,3-0-isopropylidene-/3-L-talo-pyranose. In this instance, the azide nucleophile attacks the l-O-ace-tyl group, liberating an 0-1 oxide ion which reacts with inversion of C-4. The 4-epimeric, l-O-acetyl-6-deoxy-talose derivative gives 60% of the direct inversion product l,4-anhydro-6-deoxy-2,3-0-isopropyli-dene-a-L-mannopyranose, together with other products.50... 

The fully aromatic diquaternary system 81 is prepared by acid ring closure of the salt (79) obtained by quaternization of 1,10-phen-anthroline with bromoacetaldehyde followed by dehydration of the resulting hydroxy diquaternary salt (80) with thionyl chloride.310,311 The salt 81 is unstable in aqueous solution above a pH of about 5.0. In the pH range 3.3-5.0 it is reduced by a one-electron transfer to the corresponding radical cation at a potential (E0) of —0.12 volt.311 Its reduction in dimethylformamide solution has also been studied.15,307 Substituted derivatives of 81 have been prepared.312... 

Urban et al. effected the ring closure of 2-aminopyridines and diethyl malo-nate in ethanolic sodium ethoxide. 2-Amino-4-chloropyridine and diethyl 2-methoxyethyl malonate gave only the noncyclized product (68), which was then transformed in a separate reaction with N,N -dicyclohexylcarbodiimide in dimethylformamide at room temperature to the pyrido[l,2-a]pyrimidine (69).112 Besides diethyl malonates, the active esters107 108 1 13 117 and malonyl dichlorides52,118119 have also been used as reaction partners of 2-aminopyridines. 

The reaction of disodiuni ditelluride in dimethylformamide with 2-halonitrobenzene 9 results in a displacement of the halide, reduction of the nitro group and a ring-closure reaction which affords phenazine 10 (mp 251 253 °C) in 37% yield. ... 

Schiff bases have been used in electroreductive cyclizations, leading to heterocyclic compounds. The bisimine (24), when reduced in glyme, affords the piperazine (25) in 42% yield after quenching with methyl iodide (equation 10). When the solvent is changed to dimethylformamide, saturation of the imino bonds takes place without ring closure. Transannular ring closure of diazocine (26) led to indolo-... 

In a similar mode, dimethylformamide diethylacetal condenses with the enamido-amides (262) to the corresponding dimethylamino-methyleneamides (263). On heating the latter, cyclization to the 2-pyridones (265) takes, place105 The protomeric (264) should be an intermediate if the ring-closure follows an electrocyclic course ... 

In a similar manner, acetylacetone (340) reacts with dimethylformamide-phosphorylchloride via the heptamethinium derivative (341) by double formylation and leads to 2,4-dichlorobenzaldehyde (342) (84%). 3-Penten-2-one (343a), mesityl-oxide (343 b) and 4-dimethylamino-3-penten-2-one (343 c) undergo triformylation and yield, by subsequent ring-closure, 4-chloroisophthalic dialdehydes (345a-c) ... 

Dimethylthioformamide (DMTF) (234) reacts with oxiranes (235) in the presence of a catalytic amount of trifluoroacetic acid under mild conditions to form the corresponding thiiranes in good yield as shown in Scheme 79 and Table 37 <86S779>. A possible reaction mechanism is postulated based on the mechanism observed in the preparation of oxiranes with thiocyanate or thiourea <66CRV297>. Intermediate (236) is generated by the nucleophilic attack of the sulfur atom of (234) on the carbon atom of (235) followed by ring closure reaction of the hydroxy oxygen atom with the iminium carbon atom. Subsequent elimination of dimethylformamide affords thiirane (237). 

The acrylic acid ester side chain may also be introduced by the Wittig reaction of 6-chloro-l,3-dimethyl-2,4-dioxo-l,2,3,4-tetrahydropyrimidine-5-carbaldehyde with the appropriate ylide. Subsequent conversion to a 6-amino compound and ring closure in the presence of triethyl-amine gives 8-alkyl-l,3-dimethylpyrido[2,3-t/]pyrimidine-2,4,7(l//,3//,8W)-triones 9. 45 146 Exclusive formation of pyrido[2,3-t/]pyrimidines is achieved by refluxing the acrylic acid esters in a mixture of triethylamine and dimethylformamide in the presence of l,5-diazabicyclo[4.3.0]-non-5-ene (DBN).147 55t... 

Ethyl 7-sulfanyl-2-(methylsulfanyl)-5-oxo-5,8-dihydropyrido[2,3-d]pyrimidine-6-carboxylate (7) and 1,2-dibromoethane react in dimethylformamide/potassium carbonate by additional ring closure 327 a similar cyclization has been observed with 7-amino-1,3-dimethylpyrido[2,3-d]-pyrimidine-2,4,5(l//,3tf,87/)-trione.320... 

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