The photooxygenation of chiral allylic alcohols la, (Z)-lf-k, ethers lb-d, and acetate le gave the corresponding hydroperoxy homoallylic alcohols and derivatives 2 through the ene reaction with singlet oxygen. While the reaction of the acetate le proceeded erytAro-diastereoselectively as a result of the classical cis effect, for the allylic alcohols la and (Z)-lf-k, in which an alkyl group is located cis to the hydroxy-bearing substituent, high threo selectivity was observed. This finding is explained in terms of coordination of the nucleophilic hydroxy functionality of the stereogenic center with the incoming electrophilic singlet oxygen enophile. The stereodifferentiation is a consequence of the preferred conformation of the allylic alcohol for oxygen transfer, which is mainly determined by 1,3-allylic strain, while the influence of 1,2-allylic strain is small. A similar sensitivity toward both types of allylic strain is observed in

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(dipy)Ni(COD) reagiert mit Maleinsäureanhydrid (MSA) unter Substitution zu (dipy)Ni(MSA)2 (I) und (dipy)Ni(MSA) (II). Während in I die Bindung nur über die C = C‐Gruppe erfolgt, Koordiniert MSA in II zusätzlich über eine Carbonylfunktion. Mit Phthalsäureanhydrid (PSA) und Bernsteinsäureanhydrid (BSA) ergibt (dipy)Ni(COD) im Zuge einer Redox‐addition Sechsgliedrige Metallacyclen V und VII, die bei erhöhter Temperatur unter Decarbonylierung in die fünfgliedrigen Metallacyclen VI bzw. VIII übergehen. Die IR‐Spektren der neuen Verbindungen werden diskutiert.

Ni(COD)2 reagiert in TMED mit Bernsteinsäureanhydrid unter Decarbonylierung zum (TMED) (1b). Die IR‐, 1H‐ und 13C‐NMR‐Spektren belegen, daß TMED als starker n‐Donor eine Polarisierung der Nickel‐Kohlenstoff‐σ‐Bindung bewirkt. Bei der Umsetzung von 1b mit überschüssigem CO entsteht in Methanol Bernsteinsäuremonomethylester (3).

The hydroperoxy homoallylic alcohols (2?*,5*)-2a and (S*,S*)-2&-d, readily available through the photooxygenation of chiral allylic alcohols 1, were converted into epoxy diols 4 under the catalytic action of Ti(Oi-Pr)4. In these epoxy hydroxylations, the hydroperoxides 2 play the double role as oxygen atom donor and, in form of the in situ generated corresponding unsaturated diols 3, as substrate for oxygen transfer. Compared to Ti(IV)-catalyzed epoxidations of unsaturated diols by -BuOOH, the advantage of this approach is that a large rate enhancement is obtained. Moreover, with the exception of (S*JS*)-2A, all reactions proceeded in unusually high diastereoselectivity. These results are rationalized in terms of the ability of the hydroxy-functionalized hydroperoxides (oxygen atom donors) as well as the corresponding unsaturated diols (oxygen atom acceptors) to chelate to the titanium metal in the catalytically operating template. For (S*,S*)-3a-c, bidentate binding is feasible, while for (7?*,£*)-3a and (S*,S*)-3d, this is difficult due to unfavorable steric interactions. Important for synthetic applications is the fact that allylic alcohols 1 can directly be converted into epoxy diols 4 in a one-pot, two-step procedure simply by adding catalytic amounts of Ti(Oi-Pr)4 to a photooxygenated solution of 1.Epoxy alcohols are versatile building blocks of high synthetic utility which are abundantly used in organic chemistry.1 Of the numerous methods available, metal-catalyzed epoxidations of allylic alcohols with hydroperoxides2 represent a practical route, of which the Sharpless epoxidation2d~s is undoubtedly the most valuable prototype for this purpose.An alternative, related oxyfunctionalization constitutes the direct conversion of olefins into epoxy alcohols by photooxygenation, followed by subsequent transformation of the intermediary allylic hydroperoxides under the catalytic action of transitionmetal catalysts.3 In this epoxy hydroxylation, the allylic alcohol, which is the actual epoxidized substrate, is generated in situ from the allylic hydroperoxide by oxygen transfer during the epoxidation. Hence, the allylic hydroperoxide plays the double role as oxygen donor and, after oxygen transfer, as oxygen acceptor. Therefore, no separate hydroperoxide is necessary as the oxygen atom donor reagent (Scheme I).Recently we reported that the epoxy hydroxylation methodology

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