Ester tethers

Ester-tethered enyne systems cycloisomerized to give lactone products (Eq. 11) [24]. Eor example, enyne 6 reacted under the Alder-ene conditions of [Rh(COD)Cl]2/BlNAP/ AgSbEg to give the corresponding lactone (Eq. 11). Once again free hydroxyl groups on the allylic terminus were incorporated into the cyclization precursors and subjected to the Alder-ene conditions, which led to the exclusive formation of the tautomerized products in good yields and enantioselectivities (Eq. 12). 

Gilbert and his co-workers reported that the photocycloaddition of 1-cyanonaphthalenes (260 and 262) gave 261 and 263, independently [288] (Scheme 72). In addition, they prepared and photoirradiated a variety of compounds that connected naphthalene and alkenes by methylene, ether, and ester tethers at various lengths [289], Among them, the reaction of 264 gave the best result, whereas the 2-substituted derivative (265) did not give any product. 

Scheme 19. Photoreduction of a,yS-unsaturated esters tethered to activated olefins...

Scheme 51), Weavers utilized direct formation of the a-iodomethylene-y-butyrolactone 148 from the propiolate 147 via atom transfer cyclization. Alke-noyloxymethyl iodides and selenides were converted into lactones upon treatment with tributylstannane or tributylgermane [101], In the (—)-zearalenone (152) synthesis reported by Pattenden [102] (Scheme 52), the ester tether is a bystander in the radical macrocyclization. 

Scheme 10-30 An ester tether was used to control the outcome of a [2 + 2] cycloaddition in...

The use of bis-ester tethers in remote intramolecular glycosylation has been investigated by a number of groups. In contrast to the short tethers used by Stork, Hindsgaul, and Ogawa, these more remote tethers provide a much less predictable outcome and as a result they remain less synthetically useful. Another difference between the short tethers and remote bis-ester tethers is that the reacting alcohol functionality is not masked in the tether but is already free to react as soon as the anomeric center is activated. 

Scheme 10-88 Use of phthalyl and succinyl ester tethers provides another approach to regio-and stereocontrolled glycosylation.

The formation of isomeric mixtures of cyclopropanes is not an issue in intramolecular reactions between vinylcarbenoids and dienes because the Stereochemistry of cyclopropanation is controlled by diene geometry. In the case of 102 where the double bond nearest the ester tether is trans, cyclopropanation would generate cis-divinylcyclo-propanes which would readily rearrange to cycloheptadienes (Table 15). The intramolecular cyclopropanation of trom -dienes by vinylcarbenoids is feasible even though the intermolecular cyclopropanation of a tran -alkene does not occur. Several examples of this type of intramolecular reaction are shown in Table 15. ° In contrast, when the double bond nearest the tether is cis, as with 103, the tra j-divinylcy-clopropane 104 is formed in 94% yield (Scheme 37). ... 

Other 0-heterocyclic ring formations from substructure F (Figure 6.4) concern the preparation of isochromanes [104] or the study of the effect of ether versus ester tethering during the cyclization [105]. 

Woodcock, S.R. and Branchaud, B.P. (2005) Effect of ether versus ester tethering on Heck cychzations. Tetrahedron Lett., 46, 7213 5. 

Dai and coworkers [20] studied the Wittig/intramolecular Diels-Alder (Wittig-IMDA)cycloaddition of ester-tethered 1,3,9-decatrienes 3 possessing a carbonyl substituent at Cll under controlled microwave heating in MeCN, at 180°C (Scheme 12.1). 

A gold-catalysed 3 + 2-cycloaddition-hydrolytic Michael addition-retro-aldol reaction of propargyl esters tethered to cyclohexadienones gives tetrahydrofuranones, diox-atricycloundecenones, and furofurans (Scheme 112). The product cyclohexenones or cyclohexanones with a y-quaternary centre result from multiatom transpositions... 

More effort from others has expanded the types of substrates involved in the [5+2] cycloadditions. For example, Saito and Hanzawa et al. reported the rhodium-catalyzed [5+2] cycloaddition of ester-tethered aUcyne-VCPs using fluorinated alcohols as solvents (see (14)) [27]. Mukai s work demonstrated that aUcyne-allenylcyclopropanes can also undergo rhodium-catalyzed [5+2] cycloaddition to give bicyclo[5.4.0] undecatrienes or bicyclo[5.5.0]dodecatrienes (see (15)) [35]. In related work from Yu s group, they found that the aUcene-VCP with an internal cyclopropane with the c/s-substitution can react well by following the [5+2] pathway (see (16)) while the trans isomer underwent the [3+2] process (see [3+2] section and (32)) [36]. 

Addition of dicarboethoxycarbene to cycloocta-1,3-diene yields a mixture of cis-and fran -cyclopropane adducts, probably by addition of the singlet carbene on the partially isomerized diene (due to the irradiation). Diastereoselective cyclopropan-ation of a ,/3-unsaturated acetals has been described using a camphor-derived chiral auxiliary. Intramolecular cyclopropenation of a diazo ester, tethered through a naphthalene, to an alkyne was catalysed by rhodium acetate and reported as a efficient method unfortunately, the use of chiral rhodium catalysts gave a less efficient reaction and did not provide high asymmetric induction. ... 

In the case of amide- or ester-tethered 1,6-enynes II-3, bicyclo[3.2.0]hept-6-en-2-ones have been synthesized via gold(I)-catalyzed cycloisomerization in a general way (Scheme 3.3) [8]. 

Scheme 3.3 Gold(I)-catalyzed cyclization of amide- or ester-tethered l,6,enynes 1-3...

Transition metal catalysts are also found to be effective for ring-closing reaction of ester-tethered substrates. Such ways often become valuable synthetic methodology toward bicyclic lactones. For example, Zhang and Lu [44]... 

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