Ring-opening to Acyclic Compounds

Ring-opening to Acyclic Compounds. The aziridines (277 R = Bu, 1-adamantyl) undergo selective cleavage with 2-lithio-l,3-dithian to give the respective compounds (278). °  

The AICI3-catalysed addition of C-labelled (279 R = Br) to benzene gives (280) with the label almost exclusively in the position shownThe mechanism proposed for this conversion involves a primary route via the intermediate (279 R = Ph). 

The ring-opening of aziridines by fluorinating agents yields a,j3-difluoro-amines. The stereochemistry of opening can be controlled by the correct choice of reagent, e.g. anhydrous HF, Olah s reagent, or EtaN wHF n = 2, 

° Thus (281) with anhydrous HF gives (282) (100%) whereas (283) (92%) is formed with Olah s reagent. 

Electrochemical oxidation of TV-acetyl- and A -formyl-aziridine in methanol at a platinum anode afforded MeC0NHCH2CH20Me andHCONHCH2CH2CH-(OMe)2 respectively no cyclic products were obtained.The polarographic behaviour of aziridinium salts (284 R = Ph, 4-MeC6H4, or 3- or 4-BrC6H4) in water has been reported.  

The nucleophilic opening of aziridines has also proved useful in peptide synthesis. A number of A-tosyl-aziridines (253 R = H, Me, Pr, or Ph) were opened with phenol or phenate ion in an attempt to introduce the /8-phenoxy-group to amino-acids, such groupings being known to be present in cyclopeptide alkaloids. In the case of (253 R = Ph) the ring was regioselectively cleaved to (254) (100%). 

The reaction of liquid HE with the aziridines (258 R = H, Me, Et, or Ph R, R = H, Me, or Ph) was regio- and stereo-selective, yielding the corresponding fluoro-amines (259).Thus cis- and fran.9-1,2-diphenylaziridine gave (259  

Tanaka, K. Nakajima, T. Maeda, A. Nakamura, N. Hayashi, and K. Okawa, Bull. Chem. Soc. Jpn., 

Alvernhe, E. Kozlowska-Gramz, S. Lacombe-Bar, and A. Laurent, Tetrahedron Lett., 1978, 

---

Reactions of Aziridines.- .4.1 Ring-opening to Acyclic Compounds. Aziridines are important starting materials for the synthesis of amino acids thus 3-chloroalanine (258) (39%) is obtained, (free from a-chloroalanine) by the reaction of the sodium salt of (257 R =R-R =H) in aqueous 12<. ... 

The 2/7-pyran system can undergo relatively facile electrocyclic ring opening to form m-2,4-dienones and several syntheses of 2/7-pyrans are based on the preparation of the acyclic precursors 213 in the hope that the dienone ZH-pyran equilibrium will favor the heterocycle 212. Such dienone precursors can be obtained by Knoevenagel condensation of 1,3-dicarbonyl compounds with, -unsaturated aldehydes (Scheme 118). Simple 1,3-diketones yield the ZH- pyran directly <1988IZV1815> and cyclohexan-l,3-diones afford fused pyrans <1987JOC1972>. 

Occasionally it happens that the oxo compound, produced by oxidation, forms a hydrate which is further oxidized to a dihydroxy compound. Attention must be given to the possibility (so far unreported) that when the hydrated species is in equilibrium with a trace of the ring-opened structure a sufficiently fast oxidation rate of the amino-aldehyde (i.e. the acyclic structure) could lead to the incorrect conclusion that the original material was not cyclic. 

The conversion of a 4-arylazo-5-oxazolone into a 1,2,4-triazole by reaction with a Grignard reagent is mentioned in Section II, B, 3. In HiTnilar fashion, the rearrangement of compound 30 to derivatives of 3-carboxy-l,5-diphenyl-lfl -l,2,4-triazoles (40) proceeds readily in the presence of strong nucleophiles [Eq. (26)]. This transformation undoubtedly occurs by ring opening and dehydrative cychzation, and, indeed, the acyclic amide and hydrazide 41 have been isolated. ... 

The tendency for the 3-lithio derivatives of furans and thiophenes to undergo ring opening has been exploited for the synthesis of polyunsaturated acyclic compounds. A trimethylsilyl group in the... 

The ring opening of tetrahydro-l,3-oxazines to aldehydes has recently found wide application through the work of Meyers.2-3 2-Alkylidene-tetrahydro-l,3-oxazines, prepared from the readily available 5,6-dihydro-4//-1,3-oxazines, possess strong nucleophilic properties and can react with alkyl halides and carbonyl compounds. The derivatives so obtained can be reduced to tetrahydro-l,3-oxazines, and through ring opening the latter can furnish acyclic, alicyclic, and a,jS-unsaturated aldehydes and their C-l deuterated derivatives.221-223 228... 

It was natural to turn to 5-(2-formylfuryl) methyl ether (the methyl ether of 5-hydroxymethyl-2-furaldehyde) as a possible cause of these results. This compound has an absorption peak at 284 mp. It was found82 that this compound is difficult to purify, and treatment with adsorbents leaves a bright-yellow solution. A similar color is found with 2,5-diformyl-furan, in solutions of which, color develops spontaneously on exposure to light. The dye may be closely related to the ether-dimer of 5-(hydroxy-methyl)-2-furaldehyde, perhaps with one ring open. It may be significant that development of the intense, yellow color is not observed in solutions of 5-(hydroxymethyl)-2-furaldehyde, for its acyclic form is presumably the intermediate that decomposes to levulinic and formic acids. 

---