Crystal structure of (–)-(R,E)-3-(1,3-benzodioxol-5-yl)-5-[(4S,5R)-5-hydroxymethyl-2,2-dimethyl-1,3-dioxolan-4-yl]-N,N-dimethylpent-4-enamide

In the title compound, 1,3-dioxolane and 1,3-dioxole rings adopt envelope forms, while amide moiety and benzene ring are essentially planar. An intramolecular O—H⋯O hydrogen bond supports the molecular conformation, and intermolecular weak C—H⋯O and C—H⋯π interactions connect the molecules into sheet structure.

In the title compound, C 20 H 27 NO 6 , the amide moiety is essentially planar, with a maximum deviation of 0.073 (3) Å , and one of the N-methyl groups shows rotational disorder. The five-membered 1,3-dioxolane ring adopts an envelope form, with the C atom bonded to the olefin side chain as the flap, which deviates from the mean plane through the other four atoms by 0.564 (7) Å . The 1,3dioxole ring fused to the benzene ring adopts a flattened envelope form, with the C atom between the two O atoms as the flap, which deviates from the mean plane through the other four atoms by 0.215 (7) Å . The C-C C-C olefin moiety is essentially planar and makes a dihedral angle of 87.1 (3) with the benzene ring. An intramolecular O-HÁ Á ÁO hydrogen bond supports the molecular conformation, enclosing an S(11) graph-set motif. In the crystal, intermolecular C-HÁ Á ÁO hydrogen bonding links the molecules into a tape running along the b axis. Furthermore, other weak C-HÁ Á ÁO hydrogen bonds and a C-HÁ Á Á interaction connect the tapes into a sheet structure parallel to (100).

Chemical context
Five-membered cyclic acetal is a pervasive building block in organic synthesis since it is easily prepared from an aliphatic or an aromatic 1,2-diol. These conversions are often carried out with protection of the contiguous diol (Wuts, 2014) to prevent unexpected side reactions or to reduce the polarity of the substrate, especially for carbohydrates. Although masking of the hydroxy groups is a disadvantage in terms of crystallization, due to loss of hydrogen-bond donors, it is expected to stabilize the crystal packing in order to contribute conformational rigidity by forming the cyclic acetal (Vijayasaradhi et al., 2003).
The title compound is an intermediate in the total synthesis of a natural alkaloid (Ishii et al., 2018) possessing both 1,3-ISSN 2056-9890 dioxolane and 1,3-benzodioxole components. The relative configurations were confirmed by the X-ray analysis as C7R, C10S and C14R.

Supramolecular features
The crystal packing is stabilized by a C-HÁ Á ÁO interaction (C14-H14Á Á ÁO13 i ; symmetry code as in Table 1), which links the molecules into a tape running along the b axis, with a C(3) graph-set motif. Furthermore, other weak C-HÁ Á ÁO hydrogen bonds and a C-HÁ Á Á interaction (C5-H5AÁ Á ÁO16 ii , C22-H22Á Á ÁO2 iii and C21-H21Á Á ÁCg iv ; Cg is the centroid of the C19-C24 benzene ring; Table 1) connect the tapes into a sheet parallel to (100) (Figs. 2 and 3). The molecular structure of the title compound, with the atom labelling. Displacement ellipsoids are drawn at the 50% probability levels. A yellow dotted line indicates the intramolecular O-HÁ Á ÁO hydrogen bond. Only H atoms connected to O and chiral C atoms are shown for clarity.

Figure 3
The crystal packing of the title compound, viewed down the b axis, showing layered sheet structures parallel to (100). Black dashed lines indicate intermolecular C-HÁ Á ÁO and C-HÁ Á Á interactions. Cg (green sphere) is the centroid of the C19-C24 benzene ring. Only H atoms involved in the above interactions are shown for clarity. Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
The crystal packing of the title compound, viewed down the a axis, showing the sheet structure parallel to (100). Black dashed lines indicate intermolecular C-HÁ Á ÁO and C-HÁ Á Á interactions. Cg (green sphere) is the centroid of the C19-C24 benzene ring. Only H atoms involved in the above interactions are shown for clarity. [Symmetry codes: (i) Àx + 1, y + 1 2 , Àz; (ii) x, y + 1, z; (iii) Àx + 1, y + 1 2 , Àz + 1; (iv) Àx + 1, y -1 the 1,3-dioxole rings in eight structures adopt envelope forms similar to the title compound, while those in three structures show planar (one structure has no geometrical details in the CIF).

Synthesis and crystallization
The title compound was synthesized in two steps from 3,4-Oisopropylidene-3-d-arabinopyranose (Gelas & Horton, 1975), by coupling with a known benzodioxole analogue (Rotherham & Semple, 1998) and further manipulations (Ishii et al., 2018). Purification was carried out by silica-gel column chromatography, and colourless crystals were afforded from a dichloromethane solution under a toluene-saturated atmosphere by slow evaporation at ambient temperature (m.p. 409-410 K

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The absolute configuration of the title compound was determined according to the known stereochemistries of atoms C10 and C14 derived from darabinose. The H atoms on one of the N-methyl groups (C4) are disordered; they were split into two sets of positions H4A-C and H4D-F, the refined occupancies being 0.54 (8) and 0.46 (8), respectively. C-bound H atoms were positioned geometrically, with C-H = 0.95-1.00 Å , and constrained to ride on their parent atoms, with U iso (H) = 1.5U eq (C) for methyl groups or 1.2U eq (C) otherwise. The hydroxy H atom was placed in a difference map and treated as riding, with O-H = 0.84 Å and U iso (H) = 1.5U eq (O). One problematic reflection (3,0,16) was omitted in the final refinement.    publCIF (Westrip, 2010) and PLATON (Spek, 2009).  correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.