Regioselectivity ring opening

Photochemical [2 + 2] cycloaddition is a powerful way to produce cyclobutanes, which, in turn, are reactive synthesis intermediates. N-Methylpyrrole adds aldehydes via [2 -I- 2] photocycloaddition to give transient oxetanes with high regioselectivity Ring-opening produces 3-(oi-hydroxyalkyl)pyrroles which are oxidized easily to 3-arylpyrroles, such as 3-BUTYROYL-l-METHYL-PYRROLE. With a special apparatus, ethylene is conveniently added to 3-methyl-... 

The regioselective ring-opening of vinyloxiranes by nitrogen nucleophiles offers an attractive route to vie-amino alcohols, compounds of much recent interest. As with oxygen nucleophiles, the stereochemistry of the reaction can be controlled by choice of reaction conditions aminolysis of 25, for example, affords anti-amino alcohol 26 in excellent yield and diastereoselectivity (Scheme 9.21) [48, 96, 97], and... 

A synthetically useful diastereoselectivity (90% dc) was observed with the addition of methyl-magnesium bromide to a-epoxy aldehyde 25 in the presence of titanium(IV) chloride60. After treatment of the crude product with sodium hydride, the yy -epoxy alcohol 26 was obtained in 40% yield. The yyn-product corresponds to a chelation-controlled attack of 25 by the nucleophile. Isolation of compound 28, however, reveals that the addition reaction proceeds via a regioselective ring-opening of the epoxide, which affords the titanium-complexed chloro-hydrin 27. Chelation-controlled attack of 27 by the nucleophile leads to the -syn-diastereomer 28, which is converted to the epoxy alcohol 26 by treatment with sodium hydride. 

The regioselective ring opening of the silyl epoxides is facilitated by the stabilizing effect that silicon has on a positive charge in the -position. This facile transformation permits vinylsilanes to serve as the equivalent of carbonyl groups in multistep synthesis.149... 

Heterocycles can be employed as precursors for the synthesis of pyrazoles. Pyrazoles can be synthesized by three-membered ring substrates. For example, allyl amines 12 and pyrazoles 13 could be obtained by hydrazinolysis of 2-ketoaziridines 11 <06TL255>. Regioselective ring opening of 3-aryl-2-benzoyl-l,l-cyclopropanedicarbonitriles 14 with hydrazine provided a new process for the synthesis of 5-aryl-3-phenylpyrazoles 15 <06JHC495>. 

Scheme 7-15 shows the significant improvement in overall stereoselectivity, due mainly to the adoption of the newly developed Sharpless asymmetric ep-oxidation. Compounds a-epoxy-67 and //-epoxy-67 can be readily obtained from 53 via the Sharpless reaction. Isomers of compounds 57 are then constructed via regioselective ring opening with a copper reagent. 

As with many A7(sp3)-azaheterocyclcs, monocrotaline 80 undergoes regioselective ring opening leading to 81 when treated with 2,2,2-trichloroethylchloroformate (Troc-Cl) in presence of potassium iodide (Scheme 8) <1999JA2951>. 

Scheme 5. Regioselective ring opening of alkylidenecyclopropanone acetal...

The regioselective ring-opening of epoxides 34 (R1 = Me, vinyl, Ph etc.) with aminolead compounds R23 PbNEt2, prepared from lithium diethylamide and R23 PbBr, gives good yields of the amino alcohols 3567. 

The regioselective ring-opening of the chiral epoxides 36 (R1 = Me, Pr or Ph) with aliphatic amines 37 (R2 = t-Bu, PI1CH2 or C6H13) in the presence of titanium tetraiso-propoxide leads to mixtures of the amino alcohols 38 and 39, in which the former predominate70. 

Epoxidation of amino chalcone 17, followed by regioselective ring opening of the epoxide unit, demonstrates the formation of optically active lactam derivatives of type 18, which are highly important structures for use within the pharmaceutical industry. The reaction proceeds in good yield and without loss of stereochemical integrity (Scheme 15). 

Additionally, Wu and co-workers have shown regioselective ring-opening of aziridines with acid anhydrides mediated by imidiazolinylidene 3 [157]. This pathway requires the use of an electron withdrawing tosylated aziridine 310 in order for the reaction to proceed. The mild reaction conditions allow for a variety of products to be formed in high yields (Table 25). 

Aziridines are well-known carbon electrophiles capable of reacting with various nucleophiles, and their ability to undergo regioselective ring-opening reactions... 

Regioselective ring-opening and cross-coupling metathesis of 2-substituted 7-oxanorbor-nenes (for a review of 7-oxanotbomanes and related compounds see <99T13521>) offers a new stereoselective entry into trisubstituted tetrahydrofurans <99JOC9739>. 

Regioselective ring opening of 67 at C-6 with sodium cyanide was achieved in high yield to furnish the cyanosulfate 68 <2003AGE5209>, which provides a high-yielding route to one-carbon extended carbohydrate derivatives (Equation 7). 

A -Boc-fj-methylnaphthylalanine (32) has been synthesized by stereo- and regioselective ring opening of aziridine 29 (Scheme 13)/27 This method cannot provide all four isomers of (3-methylnaphthylalanine. 

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