Four-membered heterocyclic compounds from

A recent review on four-membered heterocycles formed from imino-phosphoranes concentrates on the preparation and the reactivity of 2,4-diimino-l,3-diazetidine and related compounds (93JPR305). As an example, the synthesis via bisiminophosphorane 85 is described in Scheme 42. The bisiminophosphorane has both a heteroaryl and a styryl site. From a mechanistic view, the reaction of the bisiminophosphorane proceeds with aryl isocyanate formation via an aza-Wittig mechanism. Intermediate car-bodiimide formation (86) occurs directly on the iminophosphorane moiety... 

From Three- and Four-Membered Heterocyclic Compounds... 

Molina, P., Alajarin, M., Lopez-Leonardo, C., Elguero, J. Four-membered heterocyclic rings from iminophosphoranes. Preparation and reactivity of 2,4-dimino-1,3-diazetidines and related compounds. J. Prakt. ChemJChem.-Ztg. 1993, 335, 305-315. 

The importance of five-membered heterocycles with one heteroatom, of the benzo and dibenzo condensed systems and of the partially or completely reduced compounds as natural products, pharmaceuticals, and starting materials or auxiliaries for syntheses is much greater than for three- or four-membered heterocycles, apart from oxirane. 

There is much current interest in four-membered ring compounds containing heteroatoms that are isoelectronic with a cyclobuta-l, 3-dienediide. The first thermally robust parent homocyclic dianion [ CfSiMe,), " was only reported in 2000 the first neutral heterocyclic analogue [P(C6H2Bu 3-2,4,6) p-C( Cl) ]2, containing a Group 14 element (C) dates from 1995 Extensions to the 4" and 5" quantum group elements Ge and Sn respectively, were published in 2004. 

Saturated three- and four-membered heterocyclics absorb little in the readily accessible regions of the UV spectrum. Sulfur-containing rings are an exception, as can be seen in Table 9, Despite the lack of absorption of most parent compounds, there is a wealth of photochemistry of small heterocyclics. Light absorption by substituents, and energy transfer from photoexcited molecules present in the photoreactive system make photoconversion of the heterocycles practical. On the other hand, the lack of substantial absorption of their own can be exploited in the preparation of small heterocycles, by designing the system to be unsuitable for destructive energy transfer. 

Uncommon heterocycles, diselenaphospholes, and related compounds were reported. Reaction of the four-membered ring compound 318 with DMAD in toluene at 130 °C afforded selenadiphosphole 44, 1,2,3-diselenaphosphole 45, and 1,4-phosphaselenin derivative 319 in low yields (5-19% based on 318) (Equation 45) <2002CEJ2705>. The molecular structures of compounds 44, 45, and 319 were determined crystallographically (Section 6.12.3.1). In contrast to the mixture of heterocycles 44, 45, and 319 obtained from DMAD, compound 318 reacts with PhC=CH to give one product, 1,2,3-diselenaphosphole 78, in 58% yield (Equation 46). 

Many strained four-membered heterocycles, containing three heteroatoms, with at least one heavier group 14 element, are moisture, thermo- or light-sensitive compounds. For example, separation from the reaction mixture of 1,3,2-oxathiasiletane 63 furnished not the expected compound, but the hydrolyzed product, a thiocarbamate (Scheme 3) <2000CL244>. The adduct 64 is moisture stable and can be chromatographically purified. However, it is sensitive to gradual hydrolysis with the liberation of phenyl isothiocyanate <2002CL34>. 

The ease of ring formation helped us to make five-membered cyclic acetals from ketones and a seven-membered heterocyclic compound (see Chapter 6). In this chapter we have seen how three-membered epoxides are formed. Ring formation is generally preferred to bimolecular reactions forming open chain compounds providing that the ring is three-, five-, six-, or seven-membered. Four-membered ring are a special case discussed in Chapter 29 where all these points are developed more fully. 

The book is organized into seven chapters. In the introductory chapter, the synthetic pathways of some natural products illustrating the basic common reactions in secondary metabolites are described. In Chapter 2, methods and techniques involved in the biosynthesis of heterocycles are dealt with. The subsequent four chapters deal with three- to six-membered heterocycles starting from the natural products to approach the preparation of imnatural heterocyclic compounds with particular attention to bioactive molecules. In Chapter 7, seven- and eight-membered heterocycles are treated, as well as larger ones, using the same approach as used in the preceding four chapters. 

The simplest stable singlet carbene derived from a cyclic precursor is cyclo-propylidene. Like other carbocyclic carbenes, it does not contain a heteroatom within the ring and is therefore not further discussed here. The first cyclic carbene based on a four-membered heterocycle was described in 2004. Deprotonation of iminium salt 87a with KHMDS gave carbene dimer 88a=88a while the same reaction with 87b, bearing even bulkier N,N -substituents, led to the isolation of 88b (Scheme 1.11). Attempts to deprotonate 87a with KOt-Bu resulted in the opening of an endocyclic P-N bond, demonstrating the electrophilic character of the phosphorus atom in this compound. 

Four members of the tetraponerine family (the major constituents of the contact poison of the New Guinean ant Tetraponera sp.) were prepared by RRM methods [156]. The key step leading to tetraponerine T7 (374) from the readily available cyclopentene precursor 372 is shown in Scheme 72. When compound 372 was exposed to catalyst A in the presence of ethylene, the desired ROM-RCM sequence proceeded smoothly to furnish heterocycle 373 with complete conversion, whereas the corresponding di-nosyl (2-nitrophenylsulfonyl)-protected analog of 372 led only to a 1 2 equilibrium mixture of starting material and RRM product. 

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