Efficient approach to 3,3-bissilyl carbonyl and enol derivatives via retro-[1,4] brook rearrangement of 3-silyl allyloxysilanes

A facile and highly stereoselective retro-[1,4] Brook rearrangement of 3-silyl allyloxysilanes has been discovered. While basic hydrolysis of the formed (Z)-3,3-bissilyl lithium enolates provides 3,3-bissilyl carbonyl compounds efficiently, trapping the species with various electrophiles including alkyl halides leads to the exclusive O-substituted (Z)-3,3-bissilyl enol derivatives that can undergo a Sakurai reaction with aldehyde to produce the synthetically useful 1,2-diol diastereoselectively.     

Studies on the deprotonation and subsequent [1,4]-Wittig rearrangement of α-benzyloxyallylsilanes

Upon exposure to s-BuLi, benzyloxyallylsilane undergoes an unusually rapid and efficient [1,4]-Wittig rearrangement. Herein we describe efforts aimed at trapping the intermediate α-carbanion with an electrophile prior to rearrangement. The results of these experiments indicate that α-deprotonation and bond reorganization are separate events. Findings herein further indicate that the future success of benzyloxyallylsilanes in [1,4]-Wittig rearrangements will likely hinge on the acidity of the benzylic protons.     

Reaction of 1,4-phthalazinedione with furfural: formation of the [5,6]benza-3a,7a-diazaindane system via an unusual skeletal rearrangement

[reaction: see text] Oxidation of phthalahydrazide (1) with lead tetraacetate in the presence of furfural (3a) in methylene chloride gives [5,6]benza-3a,7a-diaza-3-carboxylindane-4,7-dione-1-ene (7a) in 64% yield. 5-Methylfurfural (3b) also reacted similarly to give the product 7b in 46% yield. Reaction of phathalazine-1,4-dione with thiophene-2-carboxaldehyde gives N-(2-formylthiophene)phthalahydrazide.     

Total synthesis of pancratistatin relying on the [3,3]-sigmatropic rearrangement

A new total synthesis of the antitumor alkaloid, pancratistatin (1), has been accomplished from readily available starting materials. Claisen rearrangement of the racemic dihydropyranethylene 17 was employed to construct the A and C rings with the appropriate stereochemistry. In addition, the Ireland-Claisen rearrangement of the enantiomerically pure 9 followed by ring-closing metathesis provided the important intermediate 24, thus implying that our approach could yield the enantioselective synthesis of (+)-pancratistatin. With the appropriately functionalized cyclohexene 24, stereo- and regiocontrolled functional group interchanges, such as iodolactonization, dihydroxylations, and a cyclic sulfate elimination reaction, facilitated the production of the target natural product.     

Synthesis and regioselectivity of the [3,3]-sigmatropic rearrangement of substituted 2-allylthio- and 2-allylseleno-1,4-dihydropyridines

The reaction of 3-cyano-1,4-dihydropyridine-2-thiolates and the corresponding selenolates with allyl bromide gave 2-allylthio- and 2-allylseleno-3-cyano-1,4-dihydropyridines, which, upon heating in various solvents or in the solid state, undergo [3,3]-sigmatropic rearrangement to give 3-cyano-3-allyl-1,2,3,4-tetrahydropyridine-2-thiones and the corresponding selenones. The resultant pyridinethiones are alkylated by alkyl halides at the sulfur atom and are oxidized by iodine to give disulfides.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1888–1895, August, 1991.     

Development of a new Lewis acid-catalyzed [3,3]-sigmatropic rearrangement: the allenoate-Claisen rearrangement

A new Lewis acid-catalyzed Claisen rearrangement has been developed that allows the stereoselective construction of beta-amino-alpha,beta,epsilon,zeta-unsaturated-gamma,delta-disubstituted esters from simple allylic amines and allenoate esters. This reaction, which is contingent upon the use of Lewis acid, can be conducted with a range of metal salts (Yb(OTf)3, AlCl3, Sn(OTf)2, Cu(OTf)2, MgBr2.Et2O, FeCl3, Zn(OTf)2) with catalyst loadings as low as 5 mol %. This catalytic process provides access to a diverse range of beta-amino-alpha,beta,epsilon,zeta-unsaturated-gamma,delta-disubstituted esters in high yield and with excellent levels of diastereoselectivity for a series of allyl pyrrolidines (R1 = H, Me, i-Pr, Ph, NR2 = pyrrolidine, piperidine, NMe2; >/=81% yield, >/=94:6 syn:anti) and allenoate esters (R2 = H, Me, i-Pr, Ph, allyl, NPht, Cl; >/=75% yield, >/=91:9 syn:anti). The capacity of this new Claisen rearrangement to provide catalytic access to elusive structural motifs has also been demonstrated in the stereospecific formation of quaternary carbon bearing frameworks arising from geranyl- and neryl pyrrolidine (>/=93% yield, >98:2 dr).     

[3,3]-sigmatropic rearrangement of allyl 2-benzothienyl sulfide

Allyl 2-benzothienyl sulfide at 20–120 °C undergoes a [3,3]-sigmatropic rearrangement to give 3-allylbenzothiophene-2-thiol. The kinetic parameters of the reaction were studied. Under the experimental conditions the thiol undergoes cyclization to give 2-methyl-2,3-dihydrobenzothieno[2,3-b]thiophene, 2-methyl-benzothieno[2,3-b]thiophene, and benzothieno[2,3-b]dihydrothiopyran. Allyl 3-methyl-2-benzothienyl sulfide does not form a thiol even at 150–190 °C but rather forms only bis(3-methyl-2-benzothienyl) disulfide.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 615–618, May, 1981.     

An unusual oxidative rearrangement of [7]-helicene

En route to helicene metal ‘sandwich’ complexes we have discovered an unusual oxidative rearrangement of [7]-helicene. The process involves a substantial skeletal rearrangement with the loss of helicity. We describe the identification of this curious product and its subsequent characterisation data.     

Studies on the [2,3]-Stevens rearrangement of aziridinium ions

The aziridinium ylide generated by the intramolecular reaction of a metal carbenoid tethered to a vinylaziridine undergoes [2,3]-Stevens rearrangement to furnish the indolizidine skeleton. It is essential that the correct nitrogen invertomer is used or a competing [1,5]-hydrogen shift predominates. During the preparation of a second system a `one-pot' acylation-[3,3]-Claisen rearrangement was observed, delivering a seven-membered lactam.     

Stereoselective [3,3]-sigmatropic rearrangement promoted by the metal [1,3]-shift of binuclear Fischer carbene complexes

Reaction of chiral homobinuclear Fischer chromium carbene complexes with allyl alcohol in the presence of NaH and the following oxidative demetalation gave alpha-allyl esters in up to 97% ee via [3,3]-sigmatropic rearrangement reaction promoted by the metal 1,3-shift. On the other hand, chiral heterobinuclear tungsten carbene complexes with arene chromium complexes afforded alpha-allyl-beta-hydroxy esters as a major product in up to 92/8 dr by the same reaction sequence.     

The zwitterion-accelerated [3,3]-sigmatropic rearrangement of allyl vinyl sulfoxided to sulfines. A specific class of charge-accelerated rearrangement

Conversion of allyl vinyl gulf oxides 5 and 8 to γ, δ-unsaturated sul fines 6 and 9, respectively, under neutral conditions shows that the accelerating effects of the charges in a zwitterionic moiety do not cancel, instead the sulfoxide functionality significantly facilitates the rearrangement.     

Studies on quinones. XV. A convenient entry into thetetrahydrophenanthrene-1,4-quinone system utilizing the dienone-phenol rearrangement of spiro [cyclopentanenaphthalene]triones

A four-step approach to the synthesis of 10-acetyloxi-5,6,7,8-tetrahydrophenanthrene-1,4-derivatives, starting from acylbenzoquinones, is described.     

Novel sultones. II. 1,4-dialkyl-9-oxo-9H-indeno [1,2-d] [1,2] oxathiin 3,3-dioxides

A new type of sultone, 1,4-dialkyl-9-oxo-9H-indeno[1,2-d] [1,2]oxathiin 3,3-dioxides, can be readily synthesized by treating 3 hydroxy-3-phenylalkanoic acid with a mixture of acid anhydride and sulfuric acid. A mechanism for the formation of these sultones is proposed. A postulated intermediate in the preparation of the 1,4-dimethyl analog, 3-ethylideneindan-1-one, was prepared and subsequently converted to the same 1,4-dimethyl compound by the same reagents. Bromination of these sultones gave the I-bromalkyl derivatives while treatment with hydrazine yielded pyrazolium sulfonates. Under different reaction conditions, 3-ethylidene-indan-1-one formed a pyrylium salt, which, in turn, was converted to a fused tricyclic pyridine derivative with ammonia.     

Monocarboxylates of 3-Methylidene-β-lactams: Synthesis and unusual oxidative rearrangement into a spiro[azetidine-3,3′-pyrrolidine] derivative

Reaction of the 3-silylated β-lactams 1 with glycoxalates gives bis-lactam 3 , but the same reaction in the presence of 1 equiv. of Me₃SiCl leads to the formation of the disilylated adducts 5 . The latter is desilylated by (Bu₄N)F yielding the monocarboxylates 7 of 3-methylidene-β-lactams, which, with oxidizing agents, give the spiro compound 8 . The structure of 8 is established by spectroscopic data and a crystal-structure analysis.     

Studies in the claisen rearrangement. Part XII . Synthesis of benzofuro[3,2-c] [l]-6a,lla-dihydro-lla-methylbenzopyrans from 1,4-diaryloxy-2-butynes

Successful Claisen rearrangement of a number of 1,4-diaryloxy-2-butynes is reported. The products of such a rearrangement are the benzofuro[3,2-c]benzopyrans. This novel rearrangement offers a facile synthetic route to tetracyclic derivatives resembling the naturally occurring pterocarpans.     

[3,3]-Sigmatropic rearrangement of allyl and 2-butenyl 1-naphthyl sulfides

The [3,3]-sigmatropic rearrangement of alkenyl 1-naphthyl sulfides in solutions with various polarities was investigated at 138–190 °C. The reaction proceeds through the formation of 2-alkenyl-1-naphthalene thiols, which subsequently undergo cyclization to compounds of the 2,3-dihydronaphtho[1,2-b]thiophene and naphtho[1,2-b]dihydrothiopyran series. 2-Butenyl 1-naphthyl sulfide, in addition to its passing directly through a [3,3]-sigmatropic rearrangement, to a considerable extent undergoes a prior [1,3]-sigmatropic rearrangement, which ultimately leads to the formation of four cyclic products. The kinetic parameters of the rearrangement of the sulfides were determined. The more negative entropies of activation constitute evidence for the high symmetry of the transition state.Translated from Khimiya Geterotskilicheskikh Soedinenii, No. 5, pp. 611–614, May, 1981.