Bicyclization of Enynes Using the Cp₂TiCl₂−Mg−BTC System:  A Practical Method to Bicyclic Cyclopentenones

Abstract

Bicyclic titanacycles 2 generated with the Cp 2 TiCl 2 -Mg-P(OEt) 3 system can be trapped with bis-(trichloromethyl) carbonate (BTC) to give bicyclic cyclopentenones 3 in good yields. The titanacycle 2m was isolated and well-identified. Bicyclization of enynes containing 1,2-disubstituted olefin by this method gave good results with excellent stereoselectivity.a Method A: CO, M ) Ti, Zr. Method B: R3SiCN, then HOAc or CuSO4, M ) Ti.

Evaluation of the Regioselectivity in Pauson-Khand Reactions of Substituted Norbornenes and Diazabicyclo[2.2.1]heptanes with Terminal Alkynes

“…Scheme 3 As shown in Scheme 3, endo-norbornene ester 9 was prepared by a thermal Diels-Alder reaction of 1-methyl-cyclopentadiene 8 [17] with maleic anhydride, followed by sub-sequent esterification. of Co 2 (CO) 8 atures (120°C) resulted in further increase in the selectivity (12e/13e 12:88, entry 11). [20] Norbornene ester 9 was then employed in NMO-promoted Pauson-Khand reactions with terminal alkynes 11 to give regioisomeric enones 12 and 13 (Scheme 4, Table 1).…”

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Keywords: Pauson-Khand reaction / Cyclopentenones 1-Methyl-norbornene ester 9 and 1-methyl-2,3-diazabicyclo[2.2.1]heptene ester 10 were employed in intermolecular Pauson-Khand reactions with various terminal alkynes 11af to give the dimethyl 1-methyltricyclo[5.2.1.0 5,9 ]dec-7-en-6one 2,3-dicarboxylates 12 and 13, and diethyl 2,3-diaza-1methyltricyclo[5.2.1.0 5,9 ]dec-7-en-6-one 2,3-dicarboxylates 14 and 15, respectively. It has also been found that other transition metals such as Fe, [2] Ru, [3] Rh, [4] Ni, [5] Cr, [6] Mo, [7] W, [8] Ti, [8] and Zr [9] can be used for the same purpose. For 2-methyl-3-bu-Transition-metal-catalysed co-cyclizations, in particular the Pauson-Khand reaction, can be considered as valuable tools in organic synthesis.

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Ni-Catalyzed Carbocyclization of 1,6-Enynes Mediated by Dialkylzinc Reagents: Me2Zn or Et2Zn Makes a Difference

“…We have previously studied the Pauson-Khand reaction of these substrates with a combination of Cp 2 TiCl 2 /Mg. 8 Herein, we report an unprecedented finding that the choice of Me 2 Zn or Et 2 Zn has dramatic influence on the reaction of 1,6-enynes with the Ni II /R 2 Zn (R = Me or Et) combination. Using a combination of different Ni II complexes and Me 2 Zn, we unexpectedly found that the [2+2+2+2]-type products 2 or the [2+2+2]-type products 3 were selectively obtained in good to excellent yields, while the reductive cyclization products 4 were exclusively formed when only 0.6 equivalents of Et 2 Zn were used as the reductant.…”

“…It is worth mentioning that excellent regioselectivity could be obtained in the formation of 3a with the two phenyl groups at meta positions, which was confirmed by X-ray crystallographic analysis. 13 Regretably, the use of solvents with weaker coordinating ability, the employment of other Ni salts or the adoption of a larger catalyst loading [20 mol% of Ni(acac) 2 ] 14 all failed to promote the formation of the [2+2+2+2]-type product 2a (Table 1, entries [4][5][6][7][8][9][10][11]. Notably, although mechanistically a catalytic amount of Me 2 Zn should be enough to effect the reaction, the reaction failed to proceed when less than 1 equivalent of Me 2 Zn was used.…”

A Titanacycle-to-Carbocycle Relay Leading to an Expedient Synthesis of Cyclopentadienols

“…Formation of the corresponding C Y X ML n ( 1 ) carbonyl compound by treatment of the metallacycles with carbon monoxide or isocyanides [1±3] and the equivalent reaction with triphosgene (OC(OCCl 3 ) 2 ) [4] should be the most representative and useful entry to this reaction [Eq. (1); ML n ZrCp 2 , TiCp 2 , Ti(OiPr) 2 ; CXY CO]. However, other variations of the reaction are notably limited.…”