A new class of bidirectionally quantitative photoswitches based on the hemithioindigo (HTI) scaffold is reported. Incorporation of a pyrrole hydrogen-bond donor leads to a bathochromic shift allowing for quantitative bidirectional isomerization. Additionally, extending conjugation from the electron-rich pyrrole results in quantitative visible-light photoswitches, as well as photoswitches that isomerize with red and near-infrared light. The presence of the hydrogen bond leading to the observed redshift is supported by computational and spectroscopic evidence.

First-row transition-metal-mediated reactions constitute an important and growing area of research due to the low cost, low toxicity, and exceptional synthetic versatility of these metals. Currently, there is considerable effort to replace existing precious-metal-catalyzed reactions with first-row analogs. More importantly, there are a plethora of unique transformations mediated by first-row metals, which have no classical second- or third-row counterpart. Herein, the application of first-row metal-mediated methods to the total synthesis of natural products is discussed. This Review is intended to highlight strategic uses of these metals to realize efficient syntheses and highlight the future potential of these reagents and catalysts in organic synthesis.

We report the total syntheses of two indole diterpenoid natural products, paspaline A and emindole PB. Paspaline A is synthesized in a 9-step sequence from commercially available materials. The first total synthesis of emindole PB is accomplished in 13 steps and confirms a previously ambiguous structural assignment. Density functional theory calculations are utilized to interrogate the key carbocationic rearrangement in a predictive capacity to aid in the selection of the most favorable precursor substrate. This work highlights how retrosynthetic design can be augmented with quantum chemical calculations to reveal energetically feasible synthetic disconnections, minimizing time-consuming and expensive empirical evaluation.

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The most red-shifted hemithioindigo photoswitches have been identified through systematic introduction of aryl units to a parent pyrrole hemithioindigo photoswitch. Increasing the size of the 5’-aryl substituent is ineffective at producing further redshifted chromophores. A second generation of 3’,5’-diarylated photoswitches which possess increased tunability is reported. Experimental and computational evidence indicates the 4’ position is electronically isolated from the bulk of the conjugated system.