SummaryThe solvolysis rates and products of several 7-anti-substituted 2-endo-norbornyl p-toluenesulfonates 11 have been determined and compared with those of the previously reported 6-exo-substituted 2-exo-norbornyl p-toluenesulfonates 1. Although the number of bonds between the substituent and the reaction site is the same in the two series, the inductive effect of the substituents is transmitted far more strongly in the 6-exo-2-exo-series 1 than in the 7-anti-2-endo-series 11; i.e. their inductivities differ widely. It is concluded that through space induction involves graded bridging of the substituent-bearing C-atom to the incipient cationic center at C (2) and that this involves differential bridging strain. The different reactivities of unsubstituted 2-exo-and 2-endo-norbornyl derivatives can then be ascribed to a stereoelectronic effect.

Plants interact with a plethora of pathogenic microorganisms in nature. Pathogen-plant interaction experiments focus mainly on single-strain infections, typically ignoring the complexity of multi-strain infections even though mixed infections are common and critical for the infection outcome. The wheat pathogenZymoseptoria triticiforms highly diverse fungal populations in which several pathogen strains often colonize the same leaf. Despite the importance of mixed infections, the mechanisms governing interactions between a mixture of pathogen strains within a plant host remain largely unexplored. Here we demonstrate that avirulent pathogen strains benefit from being in mixed infections with virulent strains. We show that virulent strains suppress the wheat immune response, allowing the avirulent strain to colonize the apoplast and to reproduce. Our experiments indicate that virulent strains in mixed infections can challenge the plant immune system both locally and systemically, providing a mechanistic explanation for the persistence of avirulent pathogen strains in fields planted to resistant host plants.

(1 1. V. 88)Further evidence is presented that the 2-norbomyl cation is stabilized primarily by C(2)-C{6) bridging, and that C(2)-C(7) bridging leads to prohibitive strain. Thus, a comparison of the heats of hydrogenation of nortricyclene 17 and bicycl0[3.2.0.0~.~]heptane 18 indicates that the strain energy of the latter is ca. 21.5 kcal/mol higher than that of 17. Furthermore, 6-exo-2-oxabicyclo[2.2. ljheptyl sulfonates 8 ionize with strong O(2) participation to the bridged oxonium ion 12. In contrast, 2-endo-7-oxabicyclo[2.2.1]heptyl sulfonates 11 ionize without O(7) participation to form the unbridged carbenium ion 15.As pointed out in [la], displacement (&2) reactions of bi-and tricyclic halides and sulfonates with nucleophilic solvents are sterically hindered, if the nucleofuge is adjacent to a bridgehead atom. In these cases, solvolysis reactions occur via carbenium ions (&1) and tend to be slow unless assisted by bridging of the cationic center by neighboring atoms.In carbocyclic compounds, bridging involves weak bonding between the electrophilic cationic center and neighboring C-atoms, thereby generating some of the strain associated with the subdivision of cyclic structures into smaller rings [2]. Differential bridging of neighboring C-atoms, therefore, accounts for the frequently different rates and products of epimeric bi-and tricyclic sulfonates [lb], such as the exo-and endo-2-norbornyl p -bromobenzenesulfonates (brosylates) and p -toluenesulfonates (tosylates) 1 (X = BsO) and 2 (X = TsO), respectively, which differ by factors of more than 3002).In their original communications [3], Winstein and Trifan attributed the higher rate of 1-OBs to anchimeric assistance of exo-ionization by the antiperiplanar C( 1)-C(6) bonding electrons, i.e. by so-called CT participation3), which is absent in the endo-epimer 2-OBs. According to a modified version of this rationale [6], the transition state for exo-ionization is stabilized by C,C hyperconjugation with the strained and antiperiplanar C( 1)-C(6) CT bond. Thus, both interpretations stress the role of the C( 1)-C(6) bonding electrons in controlling the relative rates of I-OBs and 2-OBs. On the other hand, the role of the equally strained C( 1)-C(7) bond, which is somewhat deflected from the plane ') 2, 3,The IUPAC name for norbornane: 8,9,1U-trinorbornane. The rate ratio at 25"is 350 in AcOH [3], 580 in 80% EtOH [lb], and cn. 2000 in H2O [4].For a definition of u participation, see [5].

A comparison of the solvolysis rates of the substituted 2‐exo‐ and 2‐endo ‐norbornyl p‐toluenesulfonates 1, 2, 3 and 4 and the substituted 1‐ and 2‐adamantyl sulfonates 9 and 10, respectively, in 80% ethanol and 97% trifluoroethanol has shown that the sensitivity of rates to the I‐effect of substituents, i.e. the inductivity of these compounds, varies strongly with structure, configuration and solvent. In 97% trifluoro‐ethanol, a solvent of low nucleophilicity and high ionizing power, the inductivities of the 2‐endo‐norbornyl p‐toluenesulfonates 2 and 4 as well as the inductivities of the adamantyl derivatives 9 and 10 were larger than in 80% ethanol. In contrast, the inductivity of the 2‐exo‐norbornyl p‐toluenesulfonates 1 was practically unchanged. It was, therefore, concluded that the transition states for the former compounds are not, or only weakly, bridged, whereas the transition states for the 2‐exo‐norbornyl p‐tolu‐enesulfonates 1 involve graded bridging by C (6). These results confirm that, due to differential bridging strain, 2‐norbornyl cations are anisotropic to polar effects.

The title compounds and the corresponding p -toluenesulfonates were prepared in connection with a mechanistic study of C-participation in carbocations.Fur eine Untersuchung, iiber welche gesondert berichtet wird [ 11, wurden mehrere der im Titel genannten Norbornanole la-j und 3a-j, und derdaraus erhaltlichen p-Toluolsulfonsaureester (Tosylate) 2 a-j und 4a-j, benotigt. Von diesen waren nur die 7-Chlor-[2] und die 7-Bromverbindungen lj,g und 3f,g [3], sowie das 7-Methylderivat 3 a [4] und der 7-Carbonsaureester 3d [5] bekannt. Da sich jedoch die beschriebenen Verfahren fur den vorliegenden Zweck als zu wenig ergiebig erwiesen, wurden die folgenden Synthesen ausgearbeitet.Als Ausgangsmaterial erwies sich die bekannte 2-ex0 -Brorn-5-oxonorbornan-syn -