Cyclizations, intramolecular

This Pd(0)/formic acid system was effective for the cyclization of substituted 5-allene-l-ynes to give diene 140 (Eq. 25) through initial insertion into the internal 7r-bond of the allene followed by insertion into the alkyne [79]. All of the examples provided were geminally substituted within the backbone to facilitate cyclization. Intramolecular allene-alkyne reductive couplings to generate six-membered rings were not achieved. 

Scheme 3.83. First domino radical cyclization/intramolecular Diels-Alder reaction process.

The 2-(2,4,6-triiodophenoxy)acetamide 399 on treatment with NaOMe underwent Smiles rearrangement to 400, which cyclized intramolecularly to the 2,3,5,6-tetrahydro-3-oxo-[l,4]oxazino[4,3,2-. 

Azolo[l,8]naphthyridines. The reactions of the chloronaphthyridine 229 with o-phenylenediamine at 200 °C and with sodium azide in acetic acid give the fused-ring products 230 and 231, respectively (Scheme 54) <2003IJB192>. Amino acid-substituted naphthyridines can be cyclized to the fused imidazolones 232 upon treatment with phosphorus oxychloride and under microwave irradiation (Equation 62) <2002SC857>. Acylation of 2-hydrazino-naphthyridines followed by heating gives W-acyl compounds which are cyclized intramolecularly to the [l,2,4]tri-azolo[4,3- ][l,8]naphthyridines 233 (Scheme 55) <1996IJB106>. The same compounds may also be obtained from... 

Fig. 6.25. Simplified mechanism of two degradation reactions between peptides and reducing sugars occurring in solids, a) Maillard reaction between a side-chain amino (or amido) group showing the formation of an imine (Reaction a), followed by tautomerization to an enol (Reaction b) and ultimately to a ketone (Reaction c). Reaction c is known as the Amadori rearrangement (modified from [8]). b) Postulated mechanism of the reaction between a reducing sugar and a C-terminal serine. The postulated nucleophilic addition yields an hemiacetal (Reaction a) and is followed by cyclization (intramolecular condensation Reaction b). Two subsequent hydrolytic steps (Reactions c and d) yield a serine-sugar conjugate and the des-Ser-peptide...

Waldmann et al. developed a second exo-linker following a new approach [43-44] which makes use of a safety-catch linker. It is based on the enzymatic cleavage of a functional group embodied in the linker. In this way an intermediate is generated, which subsequently cyclizes intramolecularly according to the principle of assisted removal [54—58] and thereby releases the desired target compounds (Scheme 10.11). 

Intramolecular cyclizations. Intramolecular cyclizations of electron-rich chloroacetyl indoles leads to indolo[l,2-d][l,4]benzodiazepin-6-one 20 as a result of a side nucleophilic cyclization at the indole nitrogen (Equation (4), Section 2.1.1.1 (2005T5489)). 

Gorman and Gassman905 have shown that undecatetraenes undergo cyclization (intramolecular Diels-Alder reaction) in the presence of triflic acid to provide bicyclo[4.3.0]nonyl [Eq. (5.332)], bicyclo[4.4.0]decyl, and bicyclo[5.4.0]undecyl [Eq. (5.333)] ring systems, depending on the methyl-substitution pattern. On the basis of a comparative study with varied tetraenes, they concluded that product formation, at least in some cases, could be best interpreted by a stepwise... 

The primary adduct 53 (Eq. (117) ) of the anodically generated radical R undergoes a series of follow-up reactions a) hydrogen abstraction to 54, b) dimerization to additive dimers 55, c) coupling with R to 1,2-disubsti-tuted monomers56, d) le-oxidation to a carbonium ion that either solvolyzes to 57 or, when 1,3-dicarbonyl compounds are added cyclizes intramolecularly to tetra (58) - or dihydrofuran derivatives (59). Product control is possible in some cases by suitable choice of the anode potential. With a high anode potential,... 

Weng, H., Scarlata, C., and Roth, H.D. (1996) Electron transfer photochemistry of geraniol and (E, E)-farnesol. A novel tandem , 1,5-cyclization, intramolecular capture. Journal of the American Chemical Society, 118, 10947-10953. 

Vasilenko and coworkers reported the hydrosilylation polymerization of AB2 monomers containing extremely long spacers between the SiH and vinyl groups (19)189. For the monomer with n = 10, polymers of 15,000-30,000 molecular weight were obtained in neat polymerizations. In solution, the monomer apparently cyclized intramolecularly. In the monomers with = 50 or 100, crosslinked polymers were obtained in neat polymerizations, but soluble polymers were obtained in solution polymerizations. 

A recent paper explored the 1,2-functionalization of the allene moiety in monocyclic P-lactam allenynol derivatives under palladium-catalyzed reaction conditions <02CEJ1719>. Remarkably, a novel domino process, namely allene cyclization/intramolecular Heck reaction... 

The reaction of sodium azide with 5-substituted 3-diethylamino-4-(4-methoxyphenyl)-isothiazole 1,1-dioxides afforded [l,2]thiazete 5,5-dioxides 56. It has been proposed that a nucleophilic addition of the azide ion to C-5 followed by ring closure gave a triazoline intermediate, which cyclized intramolecularly with concomitant nitrogen extrusion <02T5173>. 

For the construction of the cryptophanes depicted in Fig. 3, two different synthetic routes have been employed (Fig. 4). With reference to method (a), a suitably substituted benzyl alcohol is first converted to a C3-cyclotriveratrylene, to which three chains ending with benzyl alcoholic groups are attached and eventually cyclized intramolecularly in this reaction, the first cap serves as a template to direct the trimerization [14]. In a second, shorter route (b), a benzyl alcohol is first transformed into a dimer , which under suitable conditions trimerizes to a cryptophane in this reaction, six carbon-carbon bonds have to be created in a single sequence [24]. 

Spiroketal cyclization. Intramolecular spiroketal cyclization via Michael addition of an alcohol to a chiral a,p-unsaturated sulfoxide can proceed with high stereoselectivity. Reactions with KH are more stereoselective than those with NaH or n-BuLi. Thus cy-... 

The Weinreb group has reported the only examples of intramolecular [4 + 2] cycloadditions of iV-sulfi-nyl compounds. - For example, diene carbamate (127) could be converted to an -sulfmyl compound which cyclized intramolecularly to dihydrothiazine oxide (128) (Scheme 16). Using the chemistry outlined in equation (54), (128) was transformed stereoselectively to tAreo-sphingosine (129). Similarly, ( ,Z)-diene carbamate (130) was transformed via adduct (131) to erytAro-sphingosine (132). 

The cyclopropane ring is cleaved via the induced ring-opening of a cyclopropylcarbinyl radical to form a diradical which in turn cyclizes intramolecularly to afford the reaction product. 

Oxidation of biphenyl-2-sulfonamide with persulfate gives a sulfonamidyl radical which cyclizes intramolecularly to yield, after further oxidation. 

The present procedure starts with ethyldiphenylphosphine, which is cleaved by lithium in tetrahydrofuran to lithium ethylphenylphosphide. This phosphide under vigorous reflux cleaves tetrahydrofuran to afford a necessary precursor, 4-(ethylphenylphosphino)-1 -b utanol (ethyl(4-hydroxybutyl)pheny Iphosphine). This(hydroxyalkyl)phosphine then is cyclized intramolecularly to give 1-ethyl-l-phenylphospholanium perchlorate. 

It may, therefore, be concluded that such parameters as intramolecular cyclization, intramolecular hydrogen abstraction, steric effects, and resonance effect can most satisfactorily explain the yields of monomers in thermal degradation of polyolefins. Unfortunately, excq)t for hydrogen abstraction, three otiier parameters have not been taken into consideration in deducing expressions of rate constants for diermal d adation of polyolefins. The resonance effect should influence the activation energy of the degradation reaction and the steric effects are more prominent in the frequency factors of the rate constants ... 

The starting material is a single enantiomer and in the cyclization (intramolecular amide —-.ation) nothing happens to the one chiral centre. The reaction goes with retention of riiguration and the product is a single enantiomer too. 

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