Aza-Wittig ring closure

Aza-Wittig ring closure proved effective in making both quinoline derivative (26) <94TL(35)9229> and isoquinoline derivative (27) (Scheme 18) <95TL(36)59>. 

Oxidation of thiazolines represents another approach to thiazoles. For example, treatment of thiazoline 17, available by aza-Wittig ring closure (vide infra), with... 

PhCH2NPPh3 affords the isocyanide derivative [Os3(CO)g(CNCH2Ph)(n3-ri rri iTi -Ceo)]. The Staudinger reaction in combination with an intramolecular aza-Wittig ring closure reaction has been applied in the synthesis of quinazolin-4-ones. Other examples of intramolecular aza-Wittig reactions are reported as parts in multi-step synthetic routes. A theoretical study of the aza-Wittig reaction of HN = PX3(X = H and Cl) with formaldehyde in the gas phase and in solution has been reported. ... 

Intramolecular aza-Wittig ring closure is applied to the synthesis of thiazolines from P-azido thioesters 43, which are readily obtained from amino acid derivatives (13EJOC3290). Treatment of thioester 43 with... 

Azido esters 146, derived from a-amino acids, were subjected to intramolecular aza-Wittig ring closures to afford oxazolines 147 in good yields and with high ee values (13EJO3290). 

Another total synthesis used the rich chemistry of iminophosphoranes (95AHC159). The /3-(3-indolyl)vinyl iminophosphorane 354 underwent an aza-Wittig/electrocyclic ring closure reaction to give the carboline 355 which was hydrolyzed with lithium hydroxide (Scheme 106). A selective reduction, deprotection, decarboxylation and diazotation followed by ring closure gave Fascaplysine (353) (94TL8851). 

Of note, Quintela and coworkers used a related aza-Wittig/l,5-electrocydic ring-closure process to obtain access to multiheterocydic structural elements such as py-... 

Conjugated heterocumulenes generated in situ by an aza-Wittig reaction are also capable of electrocyclic-ring closure with a subsequent 1,3-H shift. This principle, applied for the first time by Saito et al. on butadiene iminophosphorane 51 (Scheme 28) by treatment with isocyanate and isothiocya-... 

Phosphorylated 2,3-diamino-2//-indazoles (172) are well suited for the annulation of a 1,2,4-triazole ring by a combination of an aza-Wittig and an intramolecular trapping reaction. As shown in Scheme 66, CS2 and CO2 are good reaction partners in the case of acid halides, however, no ring closure occurs, and 173 was obtained (89TL6237 90JOC4724). 

Rodrigues,]. A. R. Abramovitch, R. A. de Sousa,]. D. F. Leiva, G. C. Diastereoselective Synthesis of Cularine Alkaloids via Enium Ions and an Easy Entry to Isoquinolines by Aza-Wittig Electrocyclic Ring Closure.]. Org. Chan. 2004, 69, 2920-2928. 

The aza-Wittig reaction of iminophosphoranes 60 with isocyanates affords the carbodiimides 61, which undergo ring closure to give 2-aminopyridine derivatives 62. ... 

A -(4-Tolyl)-A -(triphenylphosphoranylidene)carboximidamide 11 reacts with aliphatic and aromatic aldehydes in refluxing xylene to give quinazolines 12 and 3,4-dihydroquinazolines 13, the ratio of which depends on the aldehyde employed. The reaction proceeds under non-acidic conditions and probably involves an aza-Wittig reaction followed by a 67t-electrocyclic ring closure of the 1,3-diazabuta-l,3-diene intermediate. Subsequent [l,5]-sigmatropic hydrogen shift results in the formation of the 3,4-dihydroquinazoline which may aromatize under the reaction conditions to yield the quinazoline 12. °... 

R = Ph, R = Et) and arylaldehydes ArCHO has been reported to yield azadienes with general formula ArC=N-C(R)=CHC02R. These azadienes appear to be very useful as starting materials for the preparation of nitrogen heterocycles. An easy one-pot synthesis of dihydropyrimidines from NH=C(Ph)N=PPh3 and a,(3-unsaturated aldehydes has been described. The reaction is supposed to proceed via an aza-Wittig mechanism followed by ring closure. ... 

Other heterocycles prepared using iminophosphoranes include novel py-rimidinone and quinazoline derivatives, obtained from in situ aza-Wittig reactions of N aryl-iV-(triphenylphosphoranylidene)carboxidamides (114) and ketenes, and pyridothienopyridazines and pyrimidothienopyridazines, accessed from aza-Wittig/electrocyclic-ring closure reactions between phos-phoranes (115) and (116) and heterocumulenes. ... 

A more recent synthesis of fascaplysin (44), from Molina and coworkers at the Universidad de Murcia, Spain, utilized a tandem aza-Wittig with electrocyclic ring-closure to generate the P-carboline ring (158). 

The condensation of iV-triphenylphosphoraniliden-benzamidine with acyclic a,P-unsaturated aldehydes produces dihydropyrimidines in good to high yields. The reaction mechanism probably involves an aza-Wittig reaction followed by a 6e-n -electrocyclic ring closure of the azatriene intermediate to give dihydropyrimidines, which is oxidized to the corresponding pyrimidines. 

The ring closure to a non-cumulated sulfoxide via an intramolecular aza-Wittig type reaction to construct S = N Unkage has been reported recently by Hemming et al. [141],... 

Molina P, Arques A, Vinader M. V. Fused pyrimidines by a tandem aza-Wittig/elec-trocyclic ring closure strategy synthesis of p5rrazolo[3,4-d]pyrimidine, [l,2,3]triazo-... 

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