Concave face

The uncoated blade showed an 0.005 inch corrosion attack over 50% of the airfoil concave face, with about 0.010 inch penetration at the base of the airfoil. Examination of the coated blade revealed no visual evidence of attack, except for one small roughened spot on the leading edge about 1 inch up from the platform, and a second spot in the middle of the convex side about 1 inch down from the tip. 

Figure 1. Convex and concave faces of a generic bicyclic system.

Our journey begins with the photo-induced union of 3,4-dimethylfuran (19) and / -(benzyloxy)-propanal (18) (see Scheme 5). Irradiation of a solution of these two simple, achiral compounds in benzene with a 450 W Hanovia lamp equipped with a vycor filter results in the exclusive formation of head-to-head, exo photoadduct 17 in 63% yield. As a cw-fused dioxabicyclo[3.2.0]heptene system, intermediate 17 possesses a folded molecular framework to which access is obstructed on the concave face. In the presence of mCPBA, the less hindered convex face of the enol ether double bond is oxidized in a completely diastereoselective fashion, affording intermediate 16 in 80% yield after regioselective opening of... 

Bergman cycloaromatization 17, 523 f., 528 biaryl coupling -.intramolecular 516f., 519 bicyclic system -, concave face 59 -, convex face 59 bicyclo[3.3.0] system 221 f. bicyclo[4.2.0] system 221 f. bidentate ligands 680 f. 

The unsubstituted pyrrolo[l,2-f]oxazol-3-one 295 underwent a dihydroxylation with catalytic osmium tetroxide to exclusively give diol 296. This diol resulted from attack of the oxidizing agent from the concave face of the molecule due to the pseudoaxial protons that sterically hinder the /3-face to be attacked by the osmium reagent (Equation 51) <2004AJC669>. 

In less repetitive syntheses, it is possible to use remote functional groups as "control elements", a technique which depends more upon the opportunist tactics developed in the course of a synthesis rather than of a premeditated strategy. Such is the case, for instance, of the synthesis of strychnine (i) by Woodward [2], in which after synthesising the intermediate 2 a hydrogen at C(8) must be introduced onto the P-face (4), i.e., onto the most hindered concave face of the molecule (Scheme 8.1). Usually the reduction with a metal hydride would lead to the a-C(8)-H isomer (i.e., the hydride ion will atack from the less hindered face of the molecule), however in the present case the P-OH group at C(21) acts as a control element and, besides the reduction of the amide at C(20), a hydride ion attacks at C(8) from the P-face by an intramolecular transfer of the complex C(21)-0-Al-H (3). 

A concave and a convex diastereoface may be distinguished, and it is apparent from ketone 7 that steric crowding is much more pronounced on the concave face. Thus, attack occurs from the convex face with high selectivity33. A variation of this principle is when the reagent adds to an unsaturated carbon at the ring fusion. 

Conventional freeze-fracture replicas revealed the presence of the two membranes enveloping the hydrogenosomes presenting a different number and distribution of intramembranous particles (Fig. 7a-c) (Benchimol et al. 1996a Benchimol 2001). Four fracture faces were identified two concave faces representing the P faces of the outer and the inner membranes and two con-... 

Figure 11.2 Three views of the cis VS ribozyme (Lipfert et al., 2008). The central view shows the concave face. The three protuberances extend from the central flat box, labeled PL, PR, and PT. PR is the most prominent. The two side views are shown in the left and right. These clearly reveal the thinness of the envelope.

Although the geometries of the corannulene moieties in these dimers could not be obtained from NMR experiments, semiempirical MNDO molecular orbital calculations clearly favor a stacked-bowl geometry (convex face to concave face). The equivalence of the four external lithium cations is explained in terms of rapid intermolecular exchange. Moreover, the equivalence of the two corannulene units is accounted for by a rapid bowl-to-bowl inversion of both corannulene decks. The validity of this proposal was supported both theoretically71 and experimentally73. 

Mehta has examined relative stereocontrol in an approach to the carbocyclic nucleus of ophiobolins. Cyclization of the triquinane (11) gave two products, (12) and (13), in a 4 1 ratio (Scheme 14). These products arise from opposite rotatory pathways as shown. Surprisingly, the major product arises from conrotation to the concave face of the diquinane unit. Finally, Nazarov has provided an interesting example of both fused and spiro mode annulations in equation (10). ... 

This is a LiAlH4 reduction of the a,(3-unsaturated ketone of the seven-membered ring. The low temperature and the use of only 0.25 eq. of LiAlH4 ensures that only the fastest reaction takes place and no reduction of the ketone in the five-membered ring or of the double bonds is observed. This reduction proceeds with substrate control of the diastereoselectivity, because the hydride attacks the molecule mainly from its convex and not from its concave face. This becomes clear when looking at 44 which is a three-dimensional representation of 31. Whether the diastereomeric ratio of 10 1 is important, will become clear in the further synthesis. 

A ligandless Heck cyclization was subsequently utilized by Hiemstra and Speckamp in their total synthesis of gelsemine (22) [5]. In this case, cyclization of 23 provided a 2 1 mixture of spirooxindoles 24/25. The lower stereoselectivity observed here is presumably due to the CHjOTDS group which adds further steric congestion to the concave face of 23. 

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