Ozonolysis stereochemistry

The early Escherunoser-Stork results indicated, that stereoselective cyclizations may be achieved, if monocyclic olefins with 1,5-polyene side chains are used as substrates in acid treatment. This assumption has now been justified by many syntheses of polycyclic systems. A typical example synthesis is given with the last reaction. The cyclization of a trideca-3,7-dien-11-ynyl cyclopentenol leads in 70% yield to a 17-acetyl A-norsteroid with correct stereochemistry at all ring junctions. Ozonolysis of ring A and aldol condensation gave dl-progesterone (M.B. Gravestock, 1978 see p. 279f.). 

An asymmetric synthesis of estrone begins with an asymmetric Michael addition of lithium enolate (178) to the scalemic sulfoxide (179). Direct treatment of the cmde Michael adduct with y /i7-chloroperbenzoic acid to oxidize the sulfoxide to a sulfone, followed by reductive removal of the bromine affords (180, X = a and PH R = H) in over 90% yield. Similarly to the conversion of (175) to (176), base-catalyzed epimerization of (180) produces an 85% isolated yield of (181, X = /5H R = H). C8 and C14 of (181) have the same relative and absolute stereochemistry as that of the naturally occurring steroids. Methylation of (181) provides (182). A (CH2)2CuLi-induced reductive cleavage of sulfone (182) followed by stereoselective alkylation of the resultant enolate with an allyl bromide yields (183). Ozonolysis of (183) produces (184) (wherein the aldehydric oxygen is by isopropyUdene) in 68% yield. Compound (184) is the optically active form of Ziegler s intermediate (176), and is converted to (+)-estrone in 6.3% overall yield and >95% enantiomeric excess (200). 

The above pathway accounts satisfactorily for the main features of ozonolysis but requires modification in detail to account for the observed stereochemistry of the reaction. Thus while a trans- (or cis-) alkene is often found to lead to a mixture of cis- and trans-ozonides as might have been expected, the trans-alkene (55) leads only to the trans-ozonide (57). The latter example demands a high degree of stereoselectivity in both the decomposition of (54) to aldehyde + peroxyzwitterion and in their subsequent recombination to (57) a demand that is not implicit in the pathway as we have written it. 

Iridium-catalyzed transfer hydrogenation of aldehyde 73 in the presence of 1,1-dimethylallene promotes tert-prenylation [64] to form the secondary neopentyl alcohol 74. In this process, isopropanol serves as the hydrogen donor, and the isolated iridium complex prepared from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and (S)-SEGPHOS is used as catalyst. Complete levels of catalyst-directed diastereoselectivity are observed. Exposure of neopentyl alcohol 74 to acetic anhydride followed by ozonolysis provides p-acetoxy aldehyde 75. Reductive coupling of aldehyde 75 with allyl acetate under transfer hydrogenation conditions results in the formation of homoallylic alcohol 76. As the stereochemistry of this addition is irrelevant, an achiral iridium complex derived from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and BIPHEP was employed as catalyst (Scheme 5.9). 

In order to establish the correct absolute stereochemistry in cyclopentanoid 123 (Scheme 10.11), a chirality transfer strategy was employed with aldehyde 117, obtained from (S)-(-)-limonene (Scheme 10.11). A modified procedure for the conversion of (S)-(-)-limonene to cyclopentene 117 (58 % from limonene) was used [58], and aldehyde 117 was reduced with diisobutylaluminium hydride (DIBAL) (quant.) and alkylated to provide tributylstannane ether 118. This compound underwent a Still-Wittig rearrangement upon treatment with n-butyl lithium (n-BuLi) to yield 119 (75 %, two steps) [59]. The extent to which the chirality transfer was successful was deemed quantitative on the basis of conversion of alcohol 119 to its (+)-(9-methyI mande I ic acid ester and subsequent analysis of optical purity. The ozonolysis (70 %) of 119, protection of the free alcohol as the silyl ether (85 %), and reduction of the ketone with DIBAL (quant.) gave alcohol 120. Elimination of the alcohol in 120 with phosphorus oxychloride-pyridine... 

It is also possible to intercept the chiral sulfoxide intermediate and convert this species to an a-amino ester. Thus, the Grignard addition to dihydro-l,2-thiazine 1-oxides 131a and 131b followed by NH4CI workup and subsequent ozonolysis of 132a and 132b affords amino ester 133 with excellent retention of the absolute stereochemistry (Scheme 17) <2004JOC7198>. 

The diozonolysis of cyclo-1,3-dienes leads to a mixture of substituted 1,2-dioxanes <1990OJO1369, 19890B145> the product composition, but not the stereochemistries of the reaction products, was determined by H and O NMR spectroscopy. 3,3-Diphenyl-l,2-dioxan-4-one was isolated and characterized by H NMR spectroscopy as one reaction product of the ozonolysis of (diphenylmethylene)cyclopropane <2001HCA1943>. 

In this section, tetramic acids with an acyl group substituent at C-3 are discussed. The simplest of the naturally occurring 3-acyl tetramic acids, tenuazonic acid (6), was first isolated from the culture filtrate of Alter-naria tenuis [18] and, subsequently, from other fungal species (A. alternate, A. longipes, Pyricularia oryzae) [19,20]. Species of Altemaria are known to produce more than 70 secondary metabolites, many of which, particularly those from the Altemaria altemata complex, are mycotoxins [19]. The absolute stereochemistry of 6 (55,65) was deduced from the formation of L-isoleucine on ozonolysis followed by acid hydrolysis [21]. 

Generally, the ozonolysis reaction shows best yields for olefins with few or small substituents. Several authors tried to define a general rule concerning the stereochemistry of this reaction without any success. The cleavage of double bonds with ozone differs greatly according to the electronic and ster-... 

The carbonyl component can be externally supplied as in the co-ozonolysis reactions (see Section 6.06.8.2) and other dipolarophiles can be used to trap the intermediate CO. Two types of rotations of the carbonyl component can take place relative to the CO <1997JOC2757> one type is in the plane of the heavy atoms which leads to the same stereochemistry as in the original alkene the other type is a rotation in a plane perpendicular to it leading to inversion. The preference of trans-alkenes to furnish in the gas phase the /raar-ozonides indicates a preference for the in-plane rotation and geminate pair recombination within the dipolar complex. At low temperatures this complex appears to be stabilized. 

One of the key steps in the synthesis of the depsipeptide jasplakinolide involved the preparation of an aldehyde 273 by ozonolysis of an alkene 271 with the correct stereochemistry of the substituents (Scheme 83). The hydroxyl group was protected with R = fer7-butyl(dimethyl)silyl <2004ASC855>. 

Opening of the fused ring of a bicyclic azetidinone has sometimes been used as a method of obtaining a monocyclic /3-lactam of known stereochemistry. Ozonolysis of the unsaturated a-D-glucopyranosylamine 149 yielded a /3-lactam ISO, which was useful in the synthesis of carbapenems and carbacephems <2000PJC1243>. 

Table 11.4 summarizes the reactions in which the electrophile and the nucleophile are linked in the same molecule (hydroboration, hydroxylation, ozonolysis). These additions occur in a concerted manner. The regiochemistry of the addition is such that the nucleophile attaches to the carbon that would be more stable as a carbocation and the addition occurs with syn stereochemistry. 

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