JSFiddle - React, Tailwind, and code Playground

HTML

<aside class="col2">
    <div class="infobox teal"> first</div>
    <div class="infobox mediumbrown">second</div>
    <div class="quotebox">third</div>
    <div class="quotebox">fourth</div>
</aside>

CSS

.last-round { border : 1px solid red }

JavaScript

$(function() {
        $("aside.col2 div:last").not(".quotebox").addClass("last-round");
    });