Other lactone formation

With nonracemic chiral diazoacetates the insertion process occurs with evident match/mismatch characteristics. This has been demonstrated in reactions of optically pure 2-methylcyclohexyl diazoacetates (Eq. 9) [85] and in carbon-hydrogen insertion reactions of steroidal diazoacetates (Eq. 10) [86], as well as with the synthesis of pyrrolizidines 36 and 37 [84]. The mechanistic preference for formation of a /J-lactone in Eq. 10 over insertion into the 4-position is not clear,but there are other examples of /J-lactone formation [87]. In these and related examples, selectivities in match/mismatch examples are high, and future investigations are anticipated to show even greater applicability. 

Other mechanisms of ketone oxidation are also known and will be discussed in Chapter 8. Peracid, which is formed from aldehyde, oxidizes ketones with lactone formation (Bayer-Villiger reaction). 

In subsequent work, Corey appUed BLA-type cationic oxazaborolidines to several other reactions, including [3 -i- 2] cycloaddition, cyanosilylation, Michael addition, and P-lactone formation. 

Several reports for the synthesis of coumarins have used lactone formation to erect the tricyclic core (Equation 44) < 1995T3197,1999SC929>. Other approaches to similar coumarins include the ring opening of an epoxide (Equation 45) <2002TA1799>. 

No O-allylation is observed in formation of the six-membered ring compound 79 by intramolecular allylation of the /i-keto ester 78[15,57], Intramolecular allylation is useful for lactone formation. On the other hand, exclusive formation of the eight-membered ring lactone 81 from 80 may be in part derived from the preference for the nucleophile to attack the less substituted terminus of the allyl system[58]. 

The rates of hydrolysis of the lactones depend on the parent structure for instance, the 1,4-lactone of D-mannonic acid is more stable than that of D-gluconic acid, and the 1,4-lactones of 2-deoxyaldonic acids are more stable than the corresponding aldonolactones. The final attainment of equilibrium between free aldonic acids and their lactones is reached only after many days at room temperature it is, however, accelerated by the presence of strong acids and by heating. A detailed discussion of the formation and hydrolysis of aldonolactones is available in a review by Shafizadeh,59 and the conformations and stabilities of aldonolactones have been discussed by Lemieux.60 Detailed analyses of D-glucono-1,5-lactone and other lactones have been reported.61 13C NMR spectroscopy proved to be a convenient method for monitoring the equilibria.62... 

A wide variety of reactions other than substitutions and hydrolyses have been performed in microemulsions. Examples include alkylations [29], Knoevenagel condensations [13], oxidations [30,31], reductions [32], formation and decomposition of Meisenheimer complexes [33], aromatic substitution reactions such as nitration and bromination [34-36], nitrosation [37] and lactone formation, i.e. esterification [38-40]. Microemulsions have also been used for photochemical and electrochemical reactions [41-45]. 

Several other methods to effect ester and lactone formation are now available. Mukaiyama uses 2-chloro-A-methylpyridinium iodide and its derivatives as a condensing agent.12 Staab s imidazole method," sue-... 

At first, both terminal double bonds of 5 are hydroborated and the diol, obtained after oxidative work-up, is treated with mild acid (PPTs) resulting in the selective protection of one OH function through lactone formation. Now, the other OH group can be selectively oxidized to give the aldehyde 16, which is then converted in a Wittig reaction to the a,/(-unsaturated ester 17. The transformation of 17 to the dialdehyde... 

In intramolecular cyclopropanation, Doyle s catalysts (159) show outstanding capabilities for enantiocontrol in the cyclization of allyl and homoallyl diazoesters to bicyclic y-and -lactones, respectively (equations 137 and 138) The data also reveal that intramolecular cyclopropanation of Z-alkenes is generally more enantioselective than that of -alkenes in bicyclic y-lactone formation . Both Rh(II)-MEPY enantiomers are available and, through their use, enantiomeric products are accessible. In a few selected cases, the Pfaltz catalyst 156 also results in high-level enantioselectivity in intramolecular cyclopropanation (equation 139) ". On the other hand, the Aratani catalyst is less effective than the Doyle catalyst (159) or Pfaltz catalyst (156) in asymmetric intramolecular cyclo-propanations In addition, the bis-oxazoline-derived copper catalyst 157b shows lower enantioselectivity in the intramolecular cyclopropanation of allyl diazomalonate (equation 140). ... 

Secondary positions tend to be more reactive towards oxidation than tertiary positions, unless steric hindrance (Uctates otherwise. Good examples of this are the chromic acid oxidation of the ether (12)4 and the ruthenium tetroxide oxidation of the ether (13), both of which lead to lactone formation (Scheme 2). Oxidation of the quassinoid intermediate (14), on the other hand, is completely nonselective (equation 15). ... 

The second key step is lactone formation from the carboethoxy substituted cyclohexanone unit in 44. The third key step is construction of the tricyclic ring system by asymmetric radical cyclization of 43, and construction of 43 from 2-isopropylphenol (42) using alcohol chiral auxiliaries (R OH) was designated as the fourth key step. This disconnection scheme represents Yang s specific approach using key chemical transformations such as radical cyclization (see sec. 13.7). Clearly, other disconnections are possible, and at each stage other disconnections could lead to alternate synthetic trees. 

In a study on Cheddar cheese, Wong et al. (1975) found that longer chain lactones (C14 to Cis) increased disproportionately to other lactones in rancid cheese. Two alternative methods of formation were proposed. The first involved the reduction of the corresponding keto acid, but investigations tended to disprove this hypothesis. The other possible method involved the microbial metabolism of homoricinoleic acid to shorter chain hydroxy acids and lactones. 

Lactonization. Lactone formation which the other propargylic position is achieved by treatment with triflic acid iC complexes are demetallated on further treatn... 

LactonizattonT Lactone formation from propargylic alcohols and homologs in which the other propargylic position is substituted with a tungsten residue is readily achieved by treatment with triflic acid (0.25 equiv) in CH Clj at -40°. The tt-allyl complexes are demetallated on further treatment with CFjCOOH. 

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