Cyclic ketones, « alkenals from

Shi and co-workers reported in 1996 the enantioselective epoxidation of unfunctionalized alkenes mediated by the fructose-derived ketone 10 (Figure 10.10) [46]. Corresponding oxiranes were obtained with excellent enantioselectivities (Equation 10.23). Yang and co-workers also reported the enantioselective epoxidation of alkenes, employing a cyclic ketone derived from binaphthyldicarboxylic add [47]. 

It has been suggested that the kinetic preference for formation of (3,y-unsaturated ketones results from an intramolecular deprotonation, as shown in the mechanism above.51 The carbonyl-ene and alkene acylation reactions have several similarities. Both reactions occur most effectively in intramolecular circumstances and provide a useful method for ring closure. Although both reactions appear to occur through highly polarized TSs, there is a strong tendency toward specificity in the proton abstraction step. This specificity and other similarities in the reaction are consistent with a cyclic formulation of the mechanism. 

Table 4.1. Rate Data for the Formation of Alkenals from Cyclic Ketones by Type I...

The introduction of umpoled synthons 177 into aldehydes or prochiral ketones leads to the formation of a new stereogenic center. In contrast to the pendant of a-bromo-a-lithio alkenes, an efficient chiral a-lithiated vinyl ether has not been developed so far. Nevertheless, substantial diastereoselectivity is observed in the addition of lithiated vinyl ethers to several chiral carbonyl compounds, in particular cyclic ketones. In these cases, stereocontrol is exhibited by the chirality of the aldehyde or ketone in the sense of substrate-induced stereoselectivity. This is illustrated by the reaction of 1-methoxy-l-lithio ethene 56 with estrone methyl ether, which is attacked by the nucleophilic carbenoid exclusively from the a-face —the typical stereochemical outcome of the nucleophilic addition to H-ketosteroids . Representative examples of various acyclic and cyclic a-lithiated vinyl ethers, generated by deprotonation, and their reactions with electrophiles are given in Table 6. 

Most photodecarbonylation reactions of cyclic ketones, especially in the vapor phase, have been postulated to proceed from various vibrational levels of excited singlet states.321 However, the elimination reaction leading to unsaturated aldehydes has now been shown to occur largely via excited triplet states. In solution, where the lowest vibrational levels of the excited states are rapidly reached, to-alkenals are the major products observed in both photolysis and radiolysis of cyclopentanone and cyclohexanone. The reaction is quenched by oxygen and dienes,322-324 as well as by the alkenal produced in the reaction.325 The reaction is also sensitized by benzene triplets.322,323 With cyclopentanone, quenching by 1M piperylene occurs some 20 times as fast... 

Homer-Wadsworth-Emmons reactions of ketones and aldehydes with phosphono-acetate esters, (R20)2P(=0)CH2C02R1, produce E/Z mixtures of a, /Tunsaturated esters. Use of the conventional reagent, sodium hydride, gives some selectivity. The combination of tin(II) triflate and A -cthylpipcndine enhances—and sometimes also reverses—the selectivity in most cases studied.71 Six-membered oxo-coordinated tin intermediates are proposed to control the selectivities observed. A similarly selective synthesis of trisubstituted exocyclic alkenes from cyclic ketones has been reported.72... 

PdCl2(PhCN)2-catalysed Claisen rearrangement of the allyl vinyl ether 474 derived from cyclic ketone at room temperature affords the syn product 475 with high diastereoselectivity [203]. In contrast to thermal Claisen rearrangement, the Pd(II)-catalysed Claisen rearrangement is always stereoselective, irrespective of the geometry of allylic alkenes. The anti product is obtained by the thermal rearrangement in the presence of 2,6-dimethylphenol at 100 °C for lOh. 

Selenadiazoles are useful intermediates for the preparation of alkenes because they can be easily decomposed with the loss of nitrogen and a selenium atom under free radical conditions. However, if 1,2,3-selenadiazoles such as 102 are treated with allyltributyl stannane/AIBN in the presence of an olefin or diene dihydroselenophenes such as 103 are formed provided the 1,2,3-selenadiazole has been derived from a cyclic ketone. Under similar conditions 1,2,3-selenadiazoles prepared from linear or aromatic ketones afford alkynes as the sole products <02JOC 1520>. 

Catalytic asymmetric epoxidation of alkenes has been achieved by means of dioxiranes formed in situ from OXONE and dioxolane-containing cyclic ketones such as 34, 35 <1999JOC6443>, and 260 <1999TA2749> derived from... 

Preformed iminium salts have been used extensively in organic synthesis. The facility of the condensation is a function of iminium salt substitution. Treatment of formaldehyde-derived methyl(methylene)ammonium halides (or trifluoroacetates) (46) with Grignard and lithium reagents results in the high yield formation of dimethylaminomethyl-containing compounds (47). Subsequent oxidation or alkylation of these products has been employed to generate terminal alkenes (48 Scheme 7). As expected, addition yields are modest for the mote-hindered iminium salts derived frrom other aldehydes and are somewhat lower for those derived from cyclic ketones. ... 

The results of irradiation ( i > 290 nm) of a series of aldehydes and ketones (91) in the presence of the silyl acetals (92) have been reported. The reactions are both solvent and silyl group dependent and the best results are obtained when the solvents used are /z-hexane, THF, diethyl ether or benzene and with the silyl group TBDMS. The products are the oxetanes (93) and the silyl-migrated product (94) in a ratio greater than 95 5 respectively. There is no evidence for the formation of the isomeric oxetane. Other studies from this research group" have examined the photochemical addition of a series of aryl aldehydes (95) to the cyclic silyl alkenes (96) brought about by irradiation at X,>290 nm in methylene chloride solution. The additions encountered take place with regio and exo selectivity as shown by the yields and ratios of the products (97). 

Pauson-Khand Cycloaddition. Pauson - Khand cycloaddition see Pauson-Khand Reaction) is a cobalt-mediated method to prepare cyclopentenone from the cyclization of an alkyne with an alkene and CO (equation 14). This method is widely used to produce cyclic ketones. Originally, stoichiometric amounts of Co2(CO)g were used in these reactions with the cobalt carbonyl being the CO source. However, it was shown that a strict temperature profile and high-purity reagents allowed the use of catalytic amounts of Co2(CO)g for reactions with 1 atm of CO. Currently, there is intense interest in developing catalytic cobalt starting materials for use in Pauson-Khand reactions. 

Fragmentation reactions may be used to prepare cyclic or acyclic alkenes from cyclic precursors. The stereochemistry of the alkene can be set up by controlling the relative stereochemistry of the cyclic substrate, a process that is normally relatively easy. The ketone 35, for example, an intermediate in a synthesis of juvenile hormone, was obtained stereospecifically from the bicyclic compound 33 using two successive... 

Dienes similarly yield cyclic ketones and bicyclic ketones can be prepared by this method. For example, the thermodynamically disfavoured tran -perhydroindanone 21 is formed on stereospecific hydroboration of 1-vinylcyclohexene, followed by carbonylation (5.32). Likewise, tra 5-1-decalone is obtained from 1-allylcyclo-hexene. The stereoselectivity of the reactions, leading exclusively to the trans fused compounds, is a consequence of the mechanism of the hydroboration, which requires syn addition of the B—H group to the double bond of the alkene. 

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